Terminal voltage matters too; higher voltage is required to deliver more power. 3.7V into 4 ohm limits you to under a watt. So many systems have a boost converter in there too - which is a potential source of electrical noise.
You could certainly have a stack of 18650 cells do the job if you want a portable boombox...
And, importantly, discharge it. Keeping a battery healthy is a moderately involved task, and the circuitry to do it is another-thing-to-go-wrong that's avoidable complexity with just about any other option. Besides which, no matter what you do the batteries will fail out within a single-digit number of years, even under low usage. It's a good option if you need portability, and pretty terrible by almost any other metric.
Terminal voltage matters too; higher voltage is required to deliver more power. 3.7V into 4 ohm limits you to under a watt. So many systems have a boost converter in there too - which is a potential source of electrical noise.
You could certainly have a stack of 18650 cells do the job if you want a portable boombox...