Or just use my xma_transform script: http://forum.xentax.com/viewtopic.php?f=17&t=9023&sid=4683927de921ba8950e3c6a695d0cdae HCS' xma parser (xma_parse.exe) needs to be in the same folder as the script.
Here's the first one (xma0) converted to WAV: https://dl.dropboxusercontent.com/u/48454461/aud/sh3_bgm_105.rar
As guest said, extract the RIFF chunk (personally I used a hex editor), run that through xmash and then towav.exe *.* with the .pos/.xma files that was generated by xmash inside of the same folder as towav.
Extract it, not cut it (i.e. put the RIFF chunk into a separate file). If you need the specs for the RIFF format, here's a spec page that's easy to understand: https://ccrma.stanford.edu/courses/422/projects/WaveFormat/
But essentially what you need to look for is "RIFF". The next four bytes after that defines how many bytes are in the RIFF chunk, so you just need to copy that amount of bytes following the field as well as those first 8 bytes ("RIFF" and chunk-size).
The chunk-size is in little-endian format, so the bytes are "reversed", so to speak. For example, little-endian 0x[AC][28][16][00] is supposed to be read 0x[00][16][28][AC] = 1452204 bytes.
You mean the _amount_ of bytes? If so: I don't know what hex editor you use, but with HxD (which I highly recommend) it's shown directly in the status bar when you put mark-up on the bytes.
If you find it complicated doing this through a hex editor, VGMToolbox can extract RIFF data (Misc. Tools -> Extraction Tools -> Generic -> Advanced Cutter/Offset Finder -> Presets: RIFF style header). That would most definitely be faster, too. I haven't tried it though...
(Not that I don't want to help you with the hex editing. Just letting you know there's probably easier ways. I just personally used a hex editor as I wasn't aware of any RIFF extraction tools at the time of making the first post)