Thank you, the new version is almost perfect! You made it! :)
I only have to report that there is sometimes some audio pops and even repeated audio (circular buffer replaying I think). However, the image never slows down (that I can notice) and the audio never goes out of sync, even with the small errors I noted.
It is curious that the filesystem type makes a difference. My data partition is PFS3AIO and here the audio skips are quite noticeable. But in my system partition which is FFS it plays very well, with only the small errors I mentioned. The most critical section is around 58 seconds when the winged character is cut in half. Tested both in emulador (drive image, not virtual folder) and real hardware (A600+IDE with CF card), the symptoms are exactly the same.
As a mere test, I tried to convert the IFF from 11khz to 8khz (sounds awful but was just a test). Used audacity to resample and ft2-clone to convert wav to iff. Unfortunately the player didn't load with this iff, so I don't know if it would have made a difference.
So, as it stands, the faster version IS playable and enjoyable on A600 with FFS partition at least. If you can squeeze just a little more bandwidth, or maybe adjust the audio buffer, it would become perfect. I should note that the "normal"/RAW version has the opposite problem: audio is perfect, but with extremely lagged video.
TL;DR: Thank you again, you cracked it! I will showcase the fast version tomorrow on our monthly gathering of Associação Inércia in Portugal :)