Debug questions – I think you have answered these, but just to be sure:
- If you load / play the fourth file first, do you still get the same error about that file, or does the new last file (e.g. NarrowMouthLoop) become the error file?
- Is one larger than the others, and do they all have the same format?
If this is a resource issue – and it sounds like it is – then it may be that this is exactly what AudioSampler is designed to deal with – memory efficient multiple simultaneous access to short sound clips. I hope that you are able to get it working with AudioPlayer, but that might turn out to be the best path.
Another option – move your Raspberry PI platform to hardware with more RAM. In testing, does that solve the problem?