Does a DC isolator smooth the voltage as well as a regulator?
Hmmm... I looked at the Murata NMG1209SC datasheet.
If I interpreted the line regulation spec properly, this is an unregulated converter. According to the datasheet the typical line regulation is 1.05 % / %. In other words, if your input voltage changes 5%, your output voltage will change by 1.05 * 5% = 5.25%. This means that if you don't have a regulator upstream, all the 120 Hz ripple from your rectifier + capacitor will make through the output of this regulator. Let's say you have 0.5V 120 Hz ripple on top of your 12V input, it gets translated into ~0.4V 120Hz ripple on the output. This sounds like a lot of buzz in your audio circuitry.

To mitigate that I would consider adding a real regulator after the rectifier and capacitor as you mentioned; cascade the isolated DC/DC with another regulator after it, as you also suggested; or I would look for a different isolated DC/DC that provides real line regulation.