Can this be also translated as "What's this/that sound?" ?
Curiously, 今の音 means that you don't hear the sound anymore when you're speaking.
When you still hear it, it'd be probably better to say, for example, これ何の音? (#3550836).
不思議だな~。。^^ And in Russian we sometimes speak about the sound we've just heard in present tense. :-)