"RIFF chunked MP3 file" is, it takes WAV format but contains MP3 data.
The applications which does't handle MP3 files (BandJAM, GDA Creator Pro etc) can handle this form.
In this chapter, I make this WAV file by SCMPX.
1. Prepare the WAV file you want to convert. Run SCMPX, and click [CONVERT] - [Single file] - [Encoding MPx] to specify that WAV file.
2. At the Encoding Settings, check "Wave header for Fhg ACM" then click [OK]. You specify the WAV file name (which contains MP3 data).
3. It start encoding. It makese WAV file (but it contains MP3 data). You can use it as the BGM file, and you can handle it by BandJAM or GDA Creator Pro etc. (It looks like WAV file for these applications.)