hey whats up i have a plan for my 8th hack and i would like to put a song in each world
EX: world 2 -when i fall in love (Remixed my own way)
world 6- red river valley
world 7-la poutie (is that it?)

anyways i just like to know how to do it like the people did in extra mario bros so i can be more crative in my hacks though
if you know pease reply.
thank you

Better late than never, I suppose... 

Frantik is correct regarding the spacial constraints- SMB1 is notorious for being short on space for expansion (musical or otherwise). That said, if your replacement songs are relatively short and/or repetitive, they can probably be worked in.

In addition to the new music code, you'll have to do some minor ASM tweaking of the music engine to load alternate songs based upon World #, as opposed to terrain/background type. That shouldn't be too difficult, depending upon your knowledge of 6502 assembly.

I assume you know enough about music to transcribe the songs you want to hack into the game; this document might be of some moderate assistance.