Those buttons are usually controlled by tact switches.
Little tiny, metal button switches that go "tick" when you press them.
Of course you don't see the switch itself unless you take the button assy. apart.
Most circuits I have seen put them on a PCB, be it a flexi board or a hard silicone layer PCB. One some the engineers put them separately from the main PCB and use a flat PCB wire ribbon cable to connect the two together. Other engineers put it on the actual PCB itself.
Just depends on the complexity of the main circuit board and unit overall, as well as what mood the engineers were in that designed it.
But, the odds are in your favor that those tact switches are on a separate PCB from the main board.
Sometimes you can get a bad solder joint at one, or both connections where the ribbon cable connects the two. Other times it actually breaks, the wire itself from movement. If it's on the "button board" side, then great, just replace that, but if it's on the main PCB side, then it has to be replaced too.
I have even seen situations where both are bad, due to bad solder joints.
If you're interested in how that happens more so these days than it ever did, read here.
Why Modern Electronics Fail
If it turns out to be just a short, or bad solder joint, or even broken ribbon cable wire. The easy thing to do is just replace the offending part.
Unless, that is, you are handy with a multi-meter, soldering iron, flux, Soldapult and wick.
