C36 and 43 are simply DC blockers. These two won't change the sound freqs.

Couple things to check. First - where are the two sets of volume adjustment jumpers? One pair for speech and one pair for sound. Are they selecting the pot's or are they selecting the two DAC functions from U13? Or hopefully not both and the digital adjustment isn't fighting with the pot adjustment.

If both sound and speech have problems -- check the area where they have shared circuitry. That would mainly be the audio amp. Sound could be distorted due to one of the two (sound/speech) inputs distorting the other.
Remove C25 and see if sound improves. If not, put C25 back and remove C34 to see if speech improves.
The U14 /U18 area is where I would focus attention.

... well, that sucks. Bad via, plated hole or trace somewhere?

Muted highs -- high frequencies are purposely clipped by the low pass filters (U13). But need to get the board working reliably before diving into this.

TP1 is your ground pin. This is the pin you use for your meter ground for all measurements. This assumes you have a good, solid ground connection to J1 pins 6 and 14.
TP2 is your main, 5V power. Measure him at a logic device such as pin 1 of U16 or pin 14 of U15.
TP3 is your 12V in... unregulated so 13.9 is ok.
TP4 is your -5V. At -4.8, he's within spec.
TP5 and TP6 control volume levels -- will be between 0 and 5 depending on where POT is turned.
TP7 Thru TP11 are all active signals which are intended to be monitored with either a logic analyzer (TP8, 10, 11) or an o'scope (TP10).
TP12 is CPU reset. A voltage lower than ~0.8 will reset the CPU, a voltage higher than ~2.4 will allow CPU to run. At 4.6 - this says reset is released (CPU allowed to run) and that level almost says your TP2 5V power is good as well. But good to measure the 5V anyways.

One place worth looking at is the condition of diodes CR2, CR3 and CR4. The sole purpose of these three is to reduce the 12V (13.9 in your case) down closer to 10V to relieve some of the heat from VR1 (LM323). They dissipate roughly 0.7 volts each with the full current load of the entire digital circuitry on them -- they can get quite hot.