If it's worth mentioning i've had Surface 2 since day one and only noticed the sound issue recently as it was fine a few days ago. I don't think is hardware related as it was ok before. Pardon the pun but 'sounds' like a driver issue to me.
I've had mine since launch and immediately noticed the tinny, bass-less sound compared to the original RT, so I don't think that's a recent driver update. Once I found this thread and disabled the "audio enhancements" the sound quality was restored but at reduced volume, and the volume drops within the first half-second of audio playback using the built-in Music and Video apps, and MediaMonkey. I don't notice the volume dropping with Flash content, however.