The PHEV has a function called Battery Saver (I think there's a + on the end of the name). It's job is to not allow the 12v battery to drain too far but using the traction battery to give it a bump when it drops to a certain level. However, it will only do that a limited number of times before it stops, and if there's still a drain on the 12v battery it will continue until it's empty. There was likely an alert on your dash when you jump started it about it being activated, but it doesn't stay on screen long.

So, how long can you sit and listen to the radio? Good question. After about 2-3 minutes of the car being in the ACC mode the car alerts you that you should "start" the car if you continue. I've always just done that, so haven't stressed the 12v that much. As you probably noticed in the specs, it's a 410CCA rating. That's rather small. If I had to guess, you might be OK after an hour, but even that might be too much. The radio itself shouldn't take too much power, but I have no idea what else might be drawing power while the car is in ACC mode, so it's possible that the draw if far greater than I'm thinking.

 

I really need to get one of those emergency power packs for jumping a dead 12v. That's so dang convenient!

 

I've had one of these in each of our cars for about 5 years & found them to be really slick and useful. Worked well for our cars & occasionally jumping other unfortunate folks. 0
Other than my 1st one dying after about 4 years, never had a failure. But you do need to recharge 'em every 4-6 months. If you don't you may not have enough oomph to get the job done.

Lots of options on Amazon, not too expensive. So much more convenient as jumper cables more uses (cell phones...), & just as effective if charged periodically. Downside is that they have seem to have a "shelf life" which jumper cables do not.