Audio system buzz with dimmer-controlled track

It may be too late given the date of this post, but the buzzing has nothing to do with the bulbs (usually, dimmers and lamp buzzing are a common problem which is why you probably got the suggestion), but in your case, you have radio-frequency interference from the dimming circuit leaking into the audio circuit. The best things you can do is either minimize the RFI, or separate the audio circuit away from the dimming circuit. Based on what you're saying about the PC input, it sounds like the main culprit is that specific audio wire going between the PC and the sound system - can it be run differently so that its further away from the dimmer/circuit. Are the PC and the dimmer on the same breaker? - If so, maybe noise is radiating from the dimmer into the PC through the power wiring - putting on different breakers could help.
One more note, the best dimmers, in terms of RFI performance, are reverse-phase control dimmers (usually designed for electronic-low-voltage... and be careful because they often get confused with magnetic low-voltage... and those will not help. Without going into too many specifics, the ELV dimmers have a much softer on/off/on/off dimming cycle when compared to standard incandescent dimmers (this is a circuitry thing with a much more complicated explanation), but as a result, they tend to have significantly fewer issues with RFI. They can be an expensive solution for simple problem solving, but the MIRELV-600 should do the trick (again, don't be fooled, the MIRLV-600 is the magnetic low-voltage product, and for what you're doing, won't help).