I had someone ask, "Why is it that my cheap $20 sound card can play more sounds than a telephony card that costs thousands of dollars?"  He was asking about a quad span T1 he purchased for his IVR application.  Basically, a T1 is an ethernet connection that can manage 24 phone lines (30 outside of the USA).  In the world of IVRs, it's a lot easier to wire, and manage, a single ethernet connection instead of wiring 24 analog phone lines.

First, let me provide some background on telephony cards.  I'll address why they're so expensive, and then I'll address the WAV part of the question.

Basically, a telephony card offloads the telephony processing from the PC's CPUs, making it easy for apps to place a call, play a sound file, collect DTMF or request speech recognition, transfer a call and so on.  Just like you have video and audio processors in your PC handling all of the video and audio signaling.

Here are some of the processing tasks handled by telephony.

Multiplexing

The phone company takes 24 phone lines assigned to you, digitizes the signal, and merges the results into one big data stream.  On the other end the telephony card takes the signal and starts breaking up that stream like a poker dealer.  Well, this dealer has 24 players to deal to.  Every few milliseconds he deals a little bit of signal to a phone line and then moves on to the next one.  Technically, this is Time-Division Multiplexing or TDM.

Signaling

After multiplexing the signal, there is additional information placed in the digitized sound stream.  Since this is a digital signal, there is no way for an IVR system to perform a "flash hook" like we do at home to answer call waiting or to make a 3-way conference call.  The telephony card and telephone switch commonly talk to each other via signalling bits.  Hidden in the sound streams are bits of information that don't affect the audio, but provide a way for the ends to pass information. 

Going off on a tangent: Without going into it too much, I just want to say that there are many standards used to perform signaling.  Don't overlook this!!!  Have a technical professional install a T1 telephony card for you.  Otherwise you'll probably spend days trying to get it to work.  Also, how your IVR handles calls and flash-hooks is different on different signaling.  Some configurations don't support flash-hook and blind transfers so you'll have to use feature codes (like dialing *29 instead) or you'll have to trombone or bridge transferred calls.

Another tangent: If you're thinking it sounds like you don't want to use a T1 and VOIP won't require all this overhead, think again!  VOIP has to handle all of this, too, but in different ways.  You can get telephony cards that handle all the VOIP processing for you, and they're probably easier to configure than a TDM T1.  If you expect to handle any kind of volume and you don't want to buy a telephony card of some sort, plan on dedicating a few high-grade CPUs for handling the telecommunications signaling and another CPU for your IVR applications. 

Advanced Features

Modern T1 cards have additional to make life easy on the main CPUs.  Echo cancellation (echoes are annoying and can confuse speech recognition, DTMF, callers...), voice activity detection (is someone on the line?), answering machine detection (each T1 card vendor has their own algorithm), and conference bridging are some of these.

To address the cost of the card

All of the processors for handling the telephony are packed on the telephony card.  When your OS loads the card it can usually provide firmware files that among other things set up the card capabilities.  So you're spending thousands of dollars on a telephony card that will do a lot more than play sound files.

Now, on to the playing of Microsoft WAV files.

Have you ever copied a song to your hard drive as a WAV file and then converted it to an MP3?  Notice the size difference?  That's because WAV files are generally the capture of the raw digital signal received from the sound device (microphone, CD player, etc.)  Just like zipping a file compresses your desktop files, a CODEC (code-decode) finds patterns in the raw sounds and compresses and encodes it into a smaller form.  It can also take an encoded file and decode it so it can be heard through your speakers or over a telephone line.

So, why by default do the boards not provide a CODEC for Microsoft WAV files?  Well, here's my guess.  Seeing that the telecommunications industry has traditionally been proprietary - not into standardizing - and each board vendor would have their own "best compression, best quality" codec.  They also provide other common industry codecs to support existing methods of encoding sound files.  When you look at the total number of codecs, frequencies and bit-rates, you can't expect a phone channel to be ready to choose any of them in real time during a live call, so you usually load some of the codecs, not all of them.  All of the supported CODECs fit into a telephony card's requirements (memory, processing power, bus bandwidth) for playing/recording sound on 24-96 simultaneous phone calls.

Microsoft WAVs are traditionally found in home and office windows-based applications.  They are for a single - or dual, stereo - sound channel, capable of higher quality than a telephone line.  There is no need for compression because the PC speakers are part of a whole computer system with excessive processing cycles and memory.

Two different industries, two different end devices, two different requirements.

Some boards may have codecs to support certain WAV formats, but if you're using uncompressed WAV files you're going to chew up a lot of disk space and use a lot of bandwidth between the sound card and the PCs disk drive (or memory cache, God willing).

To be snippy:
Microsoft Media Player doesn't play Dialogic VOX files or Natural Microsystems VOX files.  Why not?  iPods don't play WAV files.  Why not?  Everyone uses MP3s for songs, not WAVs.  Why not?  Different requirements, different priorities, different codecs.  Use the right codec for your system.

Do you have any contentions?  Please post them!  Any IVR Sales Engineers out there who want to comment?  I wouldn't mind the clarification;)  If you're looking for audio codecs for different platforms, let me know.  I have access to some proprietary formats, too.