Hi DrMapleBlade
Welcome to the Apple (user-to-user) Community.
Am no expert in this area, yet your question surely deserves a response.
Wondering if the MP3 player manufacturer may provide an app or a firmware update for use with AirPods. If may be worth checking their website, or support options.
As Apple's AirPods seem to be designed to work mainly with Apple products, third-party (non-Apple) products may not always be compatible. (Eg: Some hearing-aids are "Made for iPhone" compatible, while some are not.)
Eg: Apple Support's article: Play audio on AirPods (3rd generation) – Apple Support
Says: To protect hearing: "You can monitor and limit the volume of AirPods using your iPhone, iPad, or iPod touch".
(Eg: Maybe the volume setting needs changing / adapting for the MP3 player audio)
info: How to use: AirPods controls – Apple Support
or
How to: Change settings for AirPods Pro (all generations) – Apple Support
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In this article:
Set up AirPods with your Mac and other Bluetooth devices - Apple Support
You can scroll to: "Use AirPods with a non-Apple device"
Eg: says:
"You can use AirPods as a Bluetooth headset with a non-Apple device".Â
Hope there's something useful here, or perhaps others with more expertise than either of us may join the discussion, now it is under-way.
All the best :-)