Some people claim that sound files longer than 15 seconds cause looping problems or failures, however this is debatable since I've created personal mods which do not adhere to this limit and they've worked fine.
I would be careful to follow the format of the original WAV file though. Easiest way to do that is open a duplicate of the original file in your sound editing program and do a save as - invariably, your sound editing program will automatically choose the same settings as the file originally had, that way you will know whether to make it stereo or mono, what bit size etc.
Some sound files have multiple instances, or have variations which gives you further leeway for modding. For example, there are several 'damage creak' files (four if memory serves), this enabled me to use the one which plays at deep depths to create another mod, without totally losing creaking noises: in my case, I mixed together some sounds of whales singing and named the file as one of the damage creaks, this is actually quite a cool mod if you fancy having a go at it, as the mournful sounds whales make is actually quite creepy when you are going deep trying to sneak away on silent running.
As ever, be sure to back up the original files in case you don't like your results.