Basically, filmplay shows all names in the old DOS-style 8.3 naming format.
The GBA is not powerful enough to decode mp3 files in real-time, so you're going to have to convert in some way or another. You're right in that MusicPlayer Advance is very tedius to use, and so I've got another solution, though it does require two conversions. It's called GBA GSM Player by Tepples.
Your first step is to convert all your mp3 files to high-quality WAV files. I personally use Wavepad's batch conversion feature for this, using the "CD quality" setting so that I don't lose any audio quality in this step. From this point, you put these WAV files in the "wavs" folder and run the go.bat file. This batch file does two things: it first converts the WAV files to GSM format and sticks the *.gsm files in the "gsms" folder, then it packages whatever's in the "gsms" folder into the "gsm.gba file" produced as output. After you run the batch file, you can delete the WAV files and move the GSM files to a different folder for backup so you can use them again in other compilations if you wish.
Audio quality is obviously not as good as mp3, and it's mono only, but if you've got some background noise around you (bus, car, airplane, etc) you won't notice too much of a difference. In dead silence you will definitely be able to hear the white noise however. I was able to live with it for about a year until I got my DS and used Moonshell instead.
I never delete GSM files after I create them, as they don't take up much space at all. Heck, I've got a 2-hour long podcast that is 120 MB when in mp3 format but only 30 when in GSM format.
If that doesn't meet your specifications, then you're out of luck, as GSM Player and MusicPlayer Advance are the two main audio playback applications for the GBA.
