The question is born from:
Does taking the form of a Muse using Fey Form gain you the Sound Strike attack?.
Is 20d6 sonic damage every turn overpowered compared to other 5th- to 7th character-level abilities?
Specifically, this would be accomplished by turning into a muse using the fey form spell and obtaining the Sound Strike attack, which we can break down to 5 ranged touch attacks that deal 4d6 sonic damage each.
Best Answer
It may be achievable otherwise too, but sounds overpowered
It's hard to assess because obviously, you can not know all the cheesy or min-maxed build out there. But as a point of comparison, let's look as an actual group I know of (i.e. mine).
At level 5 to 7, a +16 to touch attack is almost guaranteed to hit. We will just flatly ignore the chance of hitting for this purpose, and assume you will land all hit, except on a 1.
20d6 average to 70 damage per turn, so 66.5 damage per turn (if you factor the crit fail).
Now, let's compare that to an actual group.
My lv 14 magus dishes out 11d6+5 (~38.5) on average, and has a hit chance of around 60% on average AC. With three attacks per turn (one gained from Haste), this amounts to roughly (60% x 38.5) x 2 + (35% x 38.5) = 59.675 damage per turn.
I'm oversimplifying the maths by not facturing the crits, but this is just a ballpark estimation.
My magus can produce that half a dozen times a day, since he is spell limited.
The other big hitters are, from bottom to top: A ranger (~65 average), a swashbuckler (~75 average) and a crit samurai (~80 average)1. All are loaded with magical items and are fairly efficient at min-maxing (it's not unusual for them to one-shot the bosses as presented in the AP).
So you would hit harder than some 14th level characters with approximately 95% chances of hitting at level 5. Ranged. At will. This is higher and more reliable than our 14th level ranger.
I'm not saying that there isn't some cheese or extremely powerful builds that can accomplish that by 5th or 7th level. But from a balance point of view, it sounds broken and overpowered. You will crush most encounters, which isn't fun nor rewarding for you or your group. My advice: don't.
This estimation is based on experience. It would be incredibly hard to produce the average damage per turn a character is supposed to output at a level, so I had to approach the problem from a simpler angle. If anyone can compute more accurate numbers, I'd be interested to see them. No doubt there are people knowledgeable enough on SE to do that.
1: This numbers are from the top of my head and rough figures. I don't know their exact average damage output, so I rounded it.