well cant tell a thing from the pics, sorry-
while a neophyte with electronics, i understand that caps can last up to twenty years...but simply , youd need to test them to be certain
presume youre tongue in cheek about wd40-it has no place on a circuit board-
i suggest, as a possibility, after the speaker - that the pot for volume control may be a likely culprit-they get the most touching, and they can either become loose in the circuit-ie a break in the joint as mentioned above, or, the pot itself can fail-
what i suggest is to power up, with adequate voltage to normally obtain sound, and then gently touch or slightly wiggle/rotate the pot shaft and see if the speaker cuts in and out-if so, then you have a wonky volume pot, or solder joint to it
if so, there are many variables -sometimes simply a loose pot cap (the round dome attacehd to the bottom plate with little tabs, which allows the wipers to miss contact, sometimes simply worn out, sometimes the wipers become loose form the shaft , etc-the thin would be to find the pot value and replace it-obviously since if youre butterfingered, you could wreck the printed circuits with too much heat, and it may be worth it to send and pay
otoh, i have done this myself, being not too clever, in a large pricey brohtal mallet, and been successful-imho the key is a 25 w soldering pen and no more, good lighting and being able to see the underside to the board so you can loate the lugs and make a clean removal-pot vlaue might be printed on the pot in the event this possibility plays out
the next thing id check would be any slide switches, or other parts which are handled, manipulated, as possible areas for solder /circuit breaks-sometimes they are virtually invisible
