How to write an XM music file format player

That's an advanced project to start with if you don't already know where to begin.

I would suggest that you start with a player that can read raw wave (PCM) files and play them, as a starting point.

Then when you have that player working well, attempt to plug in your XM code. By the time you get a wave file player running, the XM code will make more sense to you.

When I'm not sure how to start with a project like this one, I begin with the parts I do understand. And I build a project that will make troubleshooting easier.

So in that line of thinking, if you know your player is functional before you start the XM coding, then you won't have to debug that, while you're debugging you XM code. You'll have confidence in that part of the project.

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P.S. I just reread your post. The first time I thought I read that you found the XM format info, but on a reread, I think you don't.

I'm guessing that XM is a proprietary format. But it's probably based on an existing CODEC. You may need to deconstruct it on your own. You'll need to look at in a binary editor and determine what codecs it resembles. you'll do this by comparing it's header to other documented codecs. It is possible that the XM data is encrypted, which would make your effort a bit tougher.

When you get into proprietary formats, you're unlikely to be able to get much help in decoding them, unless you're willing to pay for it. It can take many hours for an experienced engineer to work them out, if the format deviates much from the standards.

Having said that, it could be as simple as being an MP3 file, with some header changes to make it incompatible with standard players.