This is really depends from the game. Usually, the graphics took most of memory (thus is compressed sometimes). Levels data could take much memory too. Digitized samples also use much memory, but that's rare for SMD game to have many samples. Music data usually very compact compared to other resources.
ROM is not reserved at all, you can use any ROM data to feed any I/O chip. VRAM is partially reserved - while tile data can go pretty much anywhere, you have to assign certain ranges of VRAM to the nametables, sprite tables, and horizontal scroll RAM. Those ranges are not hardcoded, however, and you do have some flexibility as to where they go.
If you use the Z80 exclusively for sound, then Z80 RAM will be dedicated to that purpose (although it can still read from ROM).
