Firstly, I would suggest you keep a lid on your calibration disc budget; buy too many discs and you will find yourself approaching or even exceeding the cost of a calibration meter and disc, which are excellent but relatively expensive (eg: SpectraCal, DataColor Spyder, etc ).
An option worth considering, though, is if you have buddies who are into AV, maybe you can get together and split the cost of a meter/calibration tool combo. And if so, now you are really coming up against the cost of a calibration disc or three.
Having said the above, suggested discs, about $30 each, are:
Disney WOW
Spears & Munsil HD Benchmark
JKP Digital Video Essentials: HD Basics
You say that "the audio portion is the main concern" but don't specify if you want to prioritize Multichannel Audio or 2-Channel Audio, or both. My personal opinion is you need a dedicated CD to properly setup and optimize 2-Channel Audio in contrast to a Multichannel A/V calibration disc only.
I use the following:
XLO Reference Recordings Test and Burn-In CD
Stereophile Test CD
The Sheffield Test Disc A2TB
There are others I could recommend (post back if needed) but since I feel I have enough with the above, I probably won't be using them myself. That doesn't mean there are not good options out there, though. In particular there are websites dedicated to Car Audio that offer a large number of useful audio test files to make your own Test Disc, do a little Googling and you will be rewarded.
I hope it doesn't need to be said, but I'm going to say it anyway ... no mp3's. You want at least 16 bit 44.1 Khz (same as uncompressed CD audio) files for any test or calibration files, and if your digital audio train can handle higher resolution files, well, regardless of your opinion of the value of HiRez Digital, if there was any reason to go there, calibration is that reason.
Finally, there are files you can download and use to make your own Test Disc, with the always valuable attribute of the cost being no more than one blank disk and access to an appropriate disc burner.
AVS (another audio/video website) has a free set of calibration tools:
http://www.avsforum.com/forum/139-display-calibration/948496-avs-hd-709-blu-ray-mp4-calibration.html
A list of downloadable Open Source Audio/Video and Audio Only applications (MacOS):
http://sourceforge.net/directory/science-engineering/testmeasure/os:mac/
A list of downloadable Open Source Audio/Video and Audio Only applications (WinOS):
http://sourceforge.net/directory/science-engineering/testmeasure/os:windows/
CalMAN Color Match SW (free)
http://calman.spectracal.com/calman-colormatch.html
*SpectraCAL has a number of commercial (available for purchase) A/V software tools you could consider*
Lastly, but definitely not least, by a long shot, is the venerable Room EQ Wizard software (MacOS, WinOS), which is not only excellent and well proven (its been in development for ages) but absolutely free, for the price of registering at the Home Theatre Shack (HTS) website and forum:
http://www.roomeqwizard.com
HTS Homepage:
http://www.hometheatershack.com