I've been using Emulicious for a long time to emulate GameBoy and Master System ... I didn't notice that it emulates MSX too ... 
I have rewritten the MSX code on the Sega SG-1000 - because it is a very similar system, it only has 1KB WRAM:
https://dl.dropboxusercontent.com/s/4jxilxumeyvkbjy/sg1000de...
Very nice second demo!! I like that the character knows to align itself if it's only a few pixels off, when you try to go down a path 
As for the questions:
1) For music/SFX you have two options: custom code (it's not that hard), or existing players. For SFX players, the one that ARTRAG recommended is very popular, so I think it's the best start! For Music, there's many options. For example, WYZPlayer, ArkosTracker, TriloTracker, etc. that have players. I usually use custom code, so, not sure which is best, but I played once with WYZPlayer and it's very easy to use: https://github.com/AugustoRuiz/WYZTracker
2) It depends on how do you set it up. There are two typical ways to set up 48KB ROMs: option 1: you kick the BIOS out altogether, and then you don't have access to BIOS functionality (if you are doing everything yourself, this might be ok, although maintaining 100% compatibility with the MSX standard could be tricky if that's something you want to aim for). Option 2: set the ROM as 32KB, and use the BIOS functionalities to access to the data in the first 16KB (e.g., "CALSLT"). This is what I usually do. I have a routine in the first 16KB that copies/decompresses whatever data I want from that page to RAM, and I call it using CALSLT. Then you don't need to worry about switching pages/slots, etc.