|Clockwork Quartet ... even though there's quite a lot of them in this picture!|
So, for example, The Dresden Dolls, Emilie Autumn and Jill Tracy appeared on Steampunk lists. I don't think any of those groups would say they are Steampunk, but their music obviously sounds right for the subculture. And how is that achieved? Well, that's difficult to pin down.
It seems to me that there is a clear strand of Victorian/Edwardian inspired music, stretching into the 1940s - so a lot of vaudeville and cabaret sounds, accompanied by sound effects from the early 20th century, such as broken toys, circus noises, etc. This is exemplified by the likes of Abney Park and The Dresden Dolls. There's also a range of atmospheric and ethereal sound featuring in this Steampunk collective, such as from Ethereal Mists and Jill Tracy.
Then there are some quite sonically different bands being thrown into the mix. This can be through lyrical content with clear Steampunk themes, such as 'hip hop' (or rather, 'chap hop') from the likes of Professor Elemental and Mr B The Gentleman Rhymer, who write songs about tea and daring adventures and the British aristocracy inventing mad things. But also some outliers that I've not expected, like black metal band Eibon La Furies (who are apparently inspired by Victorian themes) and Emilie Autumn (dubbed Victoriandustrial)...
It's quite interesting to see this wide collection of disparate bands being brought together by enthusiasts under a single banner. It's also nice for a genre of music to be so sonically diverse. May be that represents the diversity of the characters to be found within the subculture. Some of the bands I'm sure would be surprised to be included, but others (like Steam Powered Giraffe) are clearly targeted at that subculture. Just google Steampunk Music and search throughout the resulting lists - I hope you find something new and interesting and if so, let us know in the comments!