Presumably the music wasn’t developed in form of these source files, given that they exceed the size of C64 RAM.
I'm not sure about Ocean, but a lot of companies used the Tatung Einstein, itself a 64KiB machine, as a development platform. I would assume that the software used for building this stuff was able to deal with source files larger than the machine can hold. They might've moved onto the likes of Atari STs, IBM-compatibles, and Amigas by the time Wizball was released though. Plenty of music was developed in the form of…
The Tatung Einstein was released in 1984 in the UK, was kind of MSX-like architecturally, and used the same 3" (not 3.5") floppies as the Amstrad CPC.
I'm curious what US-based C64 devs would have used. Probably not this machine?