There's a major microphone bug on all Nexus 5s
If you look at the thread below, you'll see a google employee acknowledge there is a widespread bug/fault with the mics on the current LG Nexus 5. Symptons include hissing and the recording volume level varying widely on video recordings. It happens when there is a quiet background or low constant noise in the background.
It's not always present, it depends on the recording scenario, but it can ruin a video recording you make.
My experience is I've had some fine video recordings from the Nexus (mainly in loud situations) and others in quieter situations where the sound recording level is all over the place.
There's no point swapping your phone as they have no fix currently - ie new stock has been proven to have the same fault. They are working on an OS fix it seems (assuming that is possible). The G2 reportedly has the same problem.
This thread extensively covers the problem with examples: