It's always good to get someone to sign a release form for a couple reasons:

1) The person may not like how you put together the tracks and (depending how popular they are) tell you to take it down. Release forms let you set the conditions upon which they are releasing the audio/video to you.
2) If you get any advertising or other revenue from the video, you would need a "commercial license" to use the person's recording and not have to pay them. Releases usually have some clause saying they can use the recordings for "commercial purposes" royalty-free.

You can find general templates online and change some things to fit what you want the conditions to be. Or ask a lawyer/law student to help you out. Release forms serve to set expectations and can be persuasive to a court in lieu of privacy concerns as long as you stick to those expectations. Even without a release form, as long as you stick to expectations you should be fine.