try ebay,
When USGold converted Outrun and 720 to the 8bit machines they supplied a free audio cassette with the original arcade machine music.
Try looking for an amstrad cpc or c64 version.
I've got the outrun tape.
However, try a file sharing client as a load of arcade music is now available as MP3.
Failing that install MAME, download the 720rom and sample your sound output or use a monitor / record program.
