Combining Buck converter and LDO
It is common to follow up a buck (or boost) converter with an LDO to reduce ripple. You can even find chips that combine these two functions (buck and LDO). 

Usually microcontrollers are okay with a few 10's of mV ripple, which is achievable with a buck alone (maybe with additional output filtering).


  won't using a buck converter make ground more noisy and introduce noise to the audio signal?


This is an issue, but should be manageable if you make sure the return path for the (digital) devices powered by the high-ripple supply is well away from the (analog) devices that are noise-sensitive (assuming you're not trying to do 16-bit audio sampling or something).