Hi Scott,
I don't know if this works for you, but you could install a "dual-load" switch that would take care of the two light switches in one "gang" of the box, and then add a traditional volume control like the ones Wes suggested, or any other volume control that fits in the other "gang".
Check
http://www.lutron.com/cms400/page.aspx?id=16991&mn=1783 for some examples - the "combo" switches can be super-cool. Your local home improvement store likely also carries some simpler "toggle" style double switches.
Remember to properly isolate the electrical box so that the high-voltage and low-voltage are in their own "separated" boxes (this may or may not be possible with the current box in your wall). Combining "high" and "low" voltage in the same switchbox may be in violation of your local (and National - "NEC") electrical codes unless the devices in question have received a specific "exception" (I am not aware of any volume controls that have received this exemption - though there may be some); not to mention that such a "combination" may create a "life/safety" hazard - especially in a bathroom.
depending on the box in the wall, and where the wires enter the box, you may be able to use something like this:
http://www.hubbellcatalog.com/raco/RACO_datasheet.asp?PN=7090&FAM=RacoBoxesor:
http://www.hometech.com/hts/products/wiring/wallboxes/mixed/cn-scdiv.html(These are available at home depot.)
One last thing: Here in California, you'd have to pull a permit to do this work - check with your local building and safety department; they'll give you all the info for free. Permitting your work helps establish that the work was done according to code, and it also helps limit your future exposure in the event that some "bad" electrical thing ever happens in your house - whether as a result of your work or not.
Hope that helps,
Samson