Ok an update.....
I spoke to Fedex and they told me the RMA Return Label that Nvidia sent me can't be used for international shipment. Dulan so I chatted with the Amazon CS, told them my unit has a faulty speaker. The guy immediately apologized and arranged to have a replacement unit sent to me, no question asked!
I think I will pass either the new set or this faulty set to my mom. I have never used the external speakers before, either on this or my previous tablets. I probably won't know about the faulty speaker if not for the buzzing sound.