Hi @app.pro.englishsinqp, welcome to Thunkable! 
Remember to check out our Community Guidelines and How to ask Great Questions v2.0 posts for tips on getting the best support from the Community on your issues.
While there should be no issues uploading a .m4a asset into your project, it looks like your project has exceeded the maximium size per project of 50mb. The screenshot above is your total account storage of 500mb.
You can read more about storage limits here in our Docs.
https://docs.thunkable.com/settings/assets#app-limits
I will look into the error message you are receiving, however, as that should tell you that your project is over the size limit and not that you are uploading the wrong type of asset.