Many first effort MP3 CD players had so-so WMA support. You also may be running into the 99 track issue. Widely discussed out on the internet so I summarize and will not dive deeply into it.
If I wanted it to work, I'd stick to MP3 and 99 or less tracks in any directory.
Bob
When I make a compilation of wma tracks to play in my car using Nero the whole track wont play. I get from 5 secs to 30 secs per track. I am putting roughly 150 tracks on in one root folder. Any advice gratefull as I'm wasting a lot of discs. The first half dozen or so play right through.

Chowhound
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