Perhaps the "goblins" in the Hobbit are a smaller form of mountain-orc. There is precedent for racial variety among orcs in the LotR canon. It is also likely that they have not been under the direct thumb of Sauron for some time, and those they capture are likely to have traditions of song. Since Melkor modeled the orcs after the elves, (there is speculation that he actually twisted the Avari into orcs) it is not unreasonable to suppose them capable of twisting the joyful songs of the Children of Iluvatar and Aule into their own tunes. Plus, the orcs of Moria had drums. Considering how rarely anyone ventured into the caves, it is unlikely they built the drums solely for the purpose of scaring the socks off random passersby...
