IF you want a quick conversion nintendon's ndssndext is what you want:
http://nintendon.s6.xrea.com/
(second link down on the left or "??????")
I have a fair bit going on right now so here is a guide as quick and dirty as it comes:
At the start of the sdat file and you will see a list of ASCII (these are the file names), copy and paste into something (excel works for me).
Search for FAT (in ascii) and following that will be a list of hex numbers do a 32bit byte flip (ABCD becomes DCBA) and that is the offset of the files, the (probably 4 hex digit) number following it is the length of the file (it is not the same as the next offset minus the current one as there is padding).
Place this number list against the file names (why I said excel earlier) and you have a nice map of the sound file.
You could try overwriting what you want changed with the new file if it is smaller than the padded section (the padding can sometimes be a nice bit of extra space).
Or you can repoint to the end of the file
The value at 8 (hex) going for 4 bytes is the length of the soundfile, change this and you can now expand and add onto the end of the file.
I rather stupidly forgot the extra space details last time but they are all detailed here:
http://tahaxan.arcnor.com/index.php?option...id=38&Itemid=36
You will have to change the FILE one for simple stuff like this.