You don't normally see a lot of bard stuff for some of the more monstrous races but this is an idea I've fallen in love with. Orc Bard, drum proficient, uses clubs as mallets as a valor bard (may change when other options come along).The same concept can be used with Hobgoblins as they are a more military force. I could see a Gnoll using a drum of bone with skin stretched over it and using bones as mallets. There is just something about the idea I am loving the whole, marching horde and you hear the deep booming drumming coming from their ranks.
You can safely assume that all cultures had some variety of storytellers and/or oral history and music. If you have those things, you can make a bard for that race. The races you stated make sense from an RP perspective and can be a lot of fun to play. Some other cool "bard" ideas I've wanted to try out:
Goblin prankster/comedian. Don't even really need a weapon, but maybe a trumpet/horn that you just constantly blare to annoy people. Dwarven Lore bard who just drones on and on about the history of his clan (not the epic war ballads, but just "And Dimwin, son of Binwin, son on Bonwin, Son of.." to cast sleep or other effects.