Their voices could be heard up in our room. Here "up" is being used as:

Adverb: 
- A word that modifies something other than a noun. 
Up মূলত: Preposition হলেও এক্ষেত্রে এটি একটি Adverb হিসেবে কাজ করে। 

Up can be used in the following ways:

- as an adverb: Their voices could be heard up in our room.Mary looked up at him.I stood up.
- as a preposition: He climbed up the steps. I set off up the road.
- as an adjective: the up escalator
- after the verb ‘to be’: He was up early the next morning. Food prices are up.I knew something was up

Source: Macmillan Dictionary