a quick question

Then it helps to post such questions in the M4L forum instead of Max/MSP...

Alas, in short; First check the LOM: https://cycling74.com/docs/max5/refpages/m4l-ref/m4l_live_object_model.html

Notice that the "is_playing" property in the Song class is observable. So basically setup an observer to check if the outcome is '1' (thus the transport is playing) and if it is change the switch you're using.

For example; for a toggle to turn on you'd send a '1' into it.