First power supply on 10V output
What kind of &quot;boombox&quot; is this, that its amplifier draws only 15 mA?
I fear that you are looking at the wrong parameters of this chip. This is a linear amplifier with digital inputs.
Ivdd = 15mA is the requirement for just the logic (digital) part of this chip, whose voltags is &quot;V Logic&quot;. The amplifier part of this chip needs a lot of amperes for the audio amplifier part (PVdd in datasheet).
Look at item 7.1 of TAS6424L-Q1 amplifier's datasheet:


Definitely, this amplifier needs much more than only 15 mA.
This is a linear amplifier with digital inputs. Its audio amplifier part can support a 4.5V to 18 V power supply. There's no need to reduce the 15V input for power the amplifier, and it's not advisable for a power-hungry device like this.
It would be better if the amplifier could run directly with the rectified 15V output from transformer, and the LM317 circuit will be used only for the digital part feeding 10v to the microcontroller (I didnt't read the MCU's datasheet; does it really needs those 10V? You may need for some kind of level shifting to connect this MCU with the digital inputs of TAS6424L)
If your objective is learning, go on with your project. But if you just want to build the boombox, think about buying a switched 12V power supply.