As a guess based on only a few topside measurements on a scrap mobo, the sound comes out of UB2 and UB3 pin 11, goes to the + side of C2 and C3 which are 47 uF 16V aluminum SMT caps (could easily be faulty open or short), then proceeds out the - ends thru 180 ohms and possibly some underside EMI filtering to the switching stereo output jack, which as tomlee59 indicated could easily have an open contact.

So if you have an ohmmeter and a stereo patch cable, with power off see if you get 180 ohms from each external channel to the - side of C2 and C3 for the left and right channel outputs, and see if the + sides of C2 and C3 do connect to pin 11 of the two sound chips. With proper ohmmeter polarity directly across each electrolytic, both C2 and C3 should charge up to an open circuit (anything measuring greater than 10Kohms is fine).

If you have proper continuity, then you can try a power up test by carefully temporarily bridging C2 then C3 with an external blocking capacitor of similar value (and polarity!) in case one of these is open.

Alternatively you can patch headphones (or powered speakers) direct from ground to the - side of C2 and C3, and next if necessary use an external temporary headphone blocking capacitor of similar value and polarity connected direct to pin 11 to see if each sound chip is outputting appropriate sound voltages.