Noise levels at Trafalgar Halls PortsmouthWatch
More about the University of Portsmouth
There's no defined volume level, but you need to keep in mind that you're sharing the building with many other students, some of which may have exams the next day, be trying to study, or would just prefer some peace and quiet.
So long as you play your music at a respectful level you won't run into any problems, otherwise you'll be asked to turn it down.
Hope this helps,
~ Mikael, UoP Student Rep
It's buried in the small print, but it's there in plain English. Yours might be too.