You really should get the NTSC version and play that. Not only does it run faster than PAL (resulting in smoother gameplay), emulated NTSC versions tend to have less issues, since they are generally better emulated than their PAL variants. I don't think the all English NTSC version of Super Mario 64 will give you much trouble.
Just get the NTSC version and see if you have any more problems with the defaul audio plugin.