It was the same speakers that were used for announcements/fire-alarms/evacuations/etc. so the infrastructure was there to begin with.
I don't know what are the rules for "playing music in the office" (licensing etc. - it's not a resto, and it's only in the bathroom), but I remember 20+ years ago when I was a sys-admin tasked to find 'music for our call-center' I found some nice royalty-free music.
