It's gonna be a very difficult task, from what I can see.
1. If you are using C, you'll have to roll your own SPI code.
2. MMC involves indeterminate delays. you would have to do a lot of synchronization.
3. Before the data payload is sent, you'll probably have to issue seperate commands to the MMC and MP3 decoder. That will imply they will need to have different clocks and SO lines. But while sending the data payload, they must both share the same clock, and the SI line of the MMC will have to be connected to the SO line of the MP3 decoder.
This is getting messy. Anyway, if you are capable of such a task, you'll probably not be asking this question here. I'd advise you to seek alternative solutions.