If not for the Amiga, multimedia would not have existed until the advent of the PlayStation. The PSX borrowed more from arcade machines and high-end CGI boards than the PC, and that really turned the industry upside down to where people were programming with standard APIs (the CGI board itself), rather than hard-coding everything with the CPU in assembly. Most early CGI boards (like the one in Hard-Drivin'), were just standard paralleled CPUs rathter than purpose-built graphics chips.
I really think PC multimedia was more inspired by arcade machines and Nintendo than by the Amiga. PSX is what prompted the invention of the 3D accelerator.
Then again, remember that CD-ROMs were considered "multimedia" in the PC industry. Graphics and audio were pathetic on their own. :-)
MMX is a good example of what went wrong with the PC. Six years of work on multimedia MMX extenstions for the CPU died overnight when the 3DFX cards came out. It really is sad how long it took "custom chips" to work their way into the PC, but it would have happened eventually.