Hi,
For my build of a boombox with Up2Stream Amp 2.1 I used a battery board from Dayton Audio for 5x 18650 batteries (providing battery balance/protection and charge indicator). Fully loaded, it provides about 20,5V that falls to about 15-16V when discharging. Im quite happy with this solution. Unfortunately, I cannot provide any representative runtime statistics since it would be highly dependent on the volume used and I also do not use the boombox that often.
Regarding 12V battery: Intuitively, I would assume that the voltage is a bit too low to produce a reasonable output. Especially, if 12V is the voltage when fully loaded and it can even drop below that when discharging. Remember that power calculates as voltage^2 / resistance so that a linear increase of voltage will result in a disproportionally higher power output. Personally, I would not go below 16-18V.
Regards, Sven