As I understand it, the answer is yes it is a public performance, and yes you should have a PRS license. You can buy music which doesn't need a license. I have a number of CD's I bought from a composer in Cornwall and as he composes, produces, records it all himself, I have his express permission to use his music in my therapy sessions wherever they take place. If working in a spa, health centre, gym etc, the owners should have a license for the premises. The PRS targets specific areas of business from time to time, say garages, hairdressers, restaurants, etc. It is presumably difficult for them to police, but it is worth being sure where you stand as a therapist, were they to check your own business.