First id compress the mp3 or wav files by lowering the quality.. personally I hate music in maps, I don't mind some sfx though as long as there not overpowering... I prefer to listen to my own music during games.
Anyways, lots of programs can lower the quality of sound files.. like DB Power AMP or WinAMP... after that get a program like Pro Edit, GU Edit, or UBer@tion they all compress your map... im not sure how much they will shave off. My last map was about 160kb and compressed it was like 36 or something.. so Id guess around 1:4 compression ratio..
The main size of your maps comes from all the layers and trigger strings... so if you dont have a lot of those and only big sound files you will have to lower the quality of those to get a smaller file before using one of those programs to compress the map as those wont compress the sound files.. just the map.
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