For anyone wondering about btsnd's and loops, afaik they all loop from their loop start value to the end of the stream. The loop start value is the 32bit uint (in big endian) at 0x4 at the beginning of a btsnd.
If you don't want a loop, you just need to loop "silence".
To find the start value you want to set with your wav, either act as if you were about to make a brstm with BrawlBox (for example, if you have another free software that can do custom loops, you're free to share your knowledge) and get the number of samples for the start value or (much more "painful") try to be as precise as possible in some audio editing software (ex: Audacity) when finding the start point and multiply that value (in seconds) by 48000 (the sample rate). Then convert your number of frames in hex, for example with Windows'calc in programmer mode, open your btsnd with a hex editor such as HxD, select the block from 0x4 to 0x7 and (if your value has less than 8 chars, complete it with 0's at the beginning, ex: 296575=>0x4867f=>0x00048674) enter your start point value in hex.