We tested the MD-6 speakers on a Nokia Xpress Music 5300 and a Creative Zen Vision:M. Sound quality was very respectable on the whole. Though it didn't match the audio from Sony Ericsson's MPS-70 speakers, it was better than we expected. Our music was quite sharp and clear, and like the Nokia MD-7W speakers, it didn't sound too tinny.
You can adjust the volume only on the phone or MP3 player; there are no controls on the speakers. That's a disappointment, but it's one we can get over. The output of the speakers was more worrisome. Though the volume can get pretty loud, there's a lot of static at the higher levels. As such, the MD-6 speakers are best suited for times when you're not overly picky about how your tunes sound.
The speakers run off four AAA batteries. We'd prefer that the speakers were powered from the phone instead, but Nokia insists batteries should last for a solid 27 hours.