Beep-Media-Player displays Invalid UTF-8 when your MP3 file's ID3 title tag contains text in a non-UTF8 encoding.

If you don't want to modify the ID3 title tag you can tell BMP which encoding to use for displaying titles.

Go to the Preferences dialog and select Plugins > Media > MPEG Audio Plugin > Preferences > Title.

In the ID3 Tags configuration dialog, enable Convert non-UTF8 ID3 tags to UTF8 and enter the codeset name. For example in the ID3 encoding input box put ISO-8859-1.

Press OK and then Close to save your preferences and re-adding the files.

Link to the BMP FAQ