Y'See, everyone immediately jumps to the Plate armour example, and yes, I have seen men doing acrobatics (of a sort) in Full Plate. Chainmail, on the other hand, is very constricting around the arms and shoulders.
As for playing an instrument, most bards go for string instruments and they are definitely awkward if you can't hold them against the body.
However, it seems that WoTC are going for the bards in armour idea as of 3.5, so who am I to argue?