Probably not an quick fix but you can narrow things down..
There is probably a line out for the audio. Its low level & you can
hook it into a stereo etc & see if it sounds OK through the stereo. .
If the hum is always on the TV speakers even with the volume all the
way down its usually filter caps. See what happens & let us know.
This set is worth saving. The blue CRT face was used on all sized sets
& gave a great picture. IIRC they called it "Diamond Vision"
Latter MGA's were OK but nothing special.These were built better & with nothing left out.
Au revoir mon ami
Zeno
LFOD !