It might be possible to store an MP3 file inside of a MIDI file, but I don't think there's a standard way to do it. My guess is this device's specific MIDI + MP3 file format was only supported by this device.
As Giulio noticed, in the manual, there's a note on PDF page 20 (printed page 90) that says "For what concerns the description of the MIDI files containing MP3 files, please consult the advanced user manual." However, I can't tell if the advanced manual was ever released. The brochure PDF mentions the website www
.vsgalileo.c
om. In the Internet Archive Wayback Machine captures of that site, I found a download page, but the advanced manual is only mentioned as "available soon" up to July 2007. Then in the May 2008 capture, the advanced manual is no longer mentioned at all:
https://web.archive.org/web/20040806134305/http://www.vsgalileo.com/download.asp
https://web.archive.org/web/20070716230449/http://www.vsgalileo.com/eng/download.asp
https://web.archive.org/web/20080531081222/http://www.vsgalileo.com/eng/download.asp
In the manual, the MIDI Implementation Chart mentions a date of June 18, 2003, and the PDF properties show the manual was created July 31, 2003. So that suggests this product was released in 2003. I guess after 4 or 5 years, they might have given up on ever publishing the advanced manual?