It doesn’t make a difference in audio quality whatsoever. The difference comes in the way computations are handled by the system. Depending on the work you do with your system, it could be beneficial to make the jump to a 64bit system, especially if you ask your computer to handle a lot of processes at one time (like multiple software synths and samplers).

The work I do doesn’t require as much processing as someone who is running various virtual instruments, so I can see why people would want a 64bit operating system. For example, for me, having a 4gb RAM limitation while mastering & mixing is not a big deal and one of the advantages of a 64bit system is that it can handle much more RAM, again, useful if you’re composing and performing with loads of processes on your system.

I haven’t kept up with this discussion in a while, but last I looked, one of the major drawbacks was lack of compatible drivers for a lot of the software and firmware for peripherals that people use so before you make the jump to a 64bit system, I would advise you to make sure everything you are using is compatible (and stable) in a 64bit operating system, but with regards to the quality of audio, 64bit doesn’t offer any audible improvements. Thanks for the question and good luck!