The C64 effectively had interrupt driven loading music and the serial chip handled clocking in of the data bits and signalled an interrupt when the byte of data was ready. This meant music during loading was much easier to achieve.
I think only Firebird games had loading music on cassette. The music was composed by Melvyn Wright ([http://www.melright.com www.melright.com]). The music data was embeded into the data on cassette. The data was read as a group of 3 bits and 8 data bits. The 3 bits determined if the 8 data bits were for data or for music. The only other known software that plays music during loading from tape is [[The Mark Time Music Box]] music program.
This list is not complete, please add more. Will upload YM of the loading music soon.