I recently completed my PhD (perceptual psychology) and am really interested in doing some work in music perception in the future...struggling to gain any kind of experience or 'way in' to this field, but there is an MA course I've seen that has a lot of perfect content (Except for research methodsy content which is rather old-hand for me now).
Any advice?

On a funding basis, I would say it will be hard as you will be doing a course below your current highest achievement. But I know my supervisor did a second masters after her PhD, as part of her lecturing job, to meet a university technicality.

Though what do you want to do after the Masters? What benefit will it add and in what sector?

No, but you could just read up on it or apply for jobs in it anyway.