The support we have isn't modified to support MM10 properly. Also, the stereo thing, not true. It's all mono but there are like 5 or 4 files per song, kinda emulating the way NES sound channels were set up.
If the audio seems to be in mono when it is supposed to be in stereo and it plays at twice the speed then it is safe to assume that you are incorrectly interpreting a stereo stream as a mono stream.
It's not supposed to be stereo. Period. If it was stereo then you'd hear garbage because of interleaving issues. I think the problem is we have vgmstream locked to 44.1khz for xwb, meaning it might be sampled at 22khz.
Also, you've got the formula wrong. If it's playing as mono and double the speed, it's not stereo. Play it as stereo and you have quadruple the speed. Now, if it's stereo and playing double the speed, then it's mono. Mono and double the speed just means you need to mess with frequency.
No, using the correct samplerate, but the stereo channels being interleaved samplewise and the data being played back as mono would actually be a slowdown with 50 %.
Can't you implement support for a specific file extension like what's done for Super Paper Mario (BRSTSPM) until another better method of handling it is done?