MP3 is fine in so far as that goes, however no hardware support exists for this from any of the voice board manufacturers (namely Dialogic and NMS). When we're running 48 or 64 (and more as we move ahead) ports at the same time, having to uncompress/compress these codec in software will hammer the CPU and would quickly limit how many ports we could handle. By having the voice board hardware do it for us, we can handle a lot more calls on a single box. There are some other problems with MP3 when we looked into it and while they can be worked around, we think pushing ahead with the other codecs at our disposal (such as GSM 6.10, G.729a and G.723.1) will be a better move. They are optimized for voice (i.e. they are even more compressed than MP3), have native support by the hardware folks (including some of the fancy new IP switches poking their heads up) and will have native Windows support (since Microsoft is one of the parties pushing for support on these).
The default settings for Unity messages is 8kb/sec MuLaw which is the best sounding, but largest format. We did this since Windows natively supports it and it sounds nice. You do have the option to switch to 4kb/sec or 3kb/sec OKI ADPCM format which is much smaller, but doesn't sound quite a good. The ADPCM formats are not natively supported by Windows at this point so if they have clients without VMO that want to hear these attachments they'll have to install the codec they can get off our web page.
probably more than you were looking to get, but hope that helps.
Jeff Lindborg
Unity Product Architect
Active Voice Corp
jlindborg@activevoice.com
