You don't need a Work for Hire as they are not contributing to the song itself. However, you might want to make clear who owns the masters, recordings, mixes, etc. It might be your song, but if they did the recording, it belongs to them. Same for masters, mixes and such.
Rob, for copyright purposes, a Work for Hire is where you pay someone to provide you with content yet you keep the copyright. An advertising agency might be a good example; where they contact me to write a jingle for a product to air on the radio, TV, etc. I wrote the song, but because they hired me to do it, they own the rights to it. For that to happen, there has to be an executed contract, blah, blah, blah.