Just noticed something: Had music playing over the speakers, sounded tinny and poor as you describe. Plugged in headphones and I heard a distinct change from tinny and poor sound to rich and full sound. As though when the plug was put in all the way the machine detected the difference and had a delayed reaction in how it processed the music.
Also, the volume rocker on the side of the Surface quit working for a bit but I could adjust it with the toggle within the Xbox music app. Physical rocker had the volume at 0, Xbox volume had it at 80. Then, things switched again and suddenly the physical rocker is working again (happened when a new song came on) and the Xbox toggle switched to matching the hardware rocker again. Weird.