Bob Parnass published a mod for the PRO-43
years ago that provided a significant boost to
the high-frequency response of the audio amp.
A lot of scanners' audio is too bassy. To get
decent volume, you have to turn it up and then
the excess bass overloads the amp. By boosting
the highs, you don't need to use as high of
a setting to get the same perceived volume.

What the mod did was to remove a bypass cap
from the input of the audio amp chip, thereby
letting the amp chip amplify the high frequencies
that would never normally make it through
to the speaker.

If you can find the amp chip in your scanner, do
a google search for the chip number. Get the
pinouts. Look for a bypass cap on the input of
the audio amp chip and remove it. The cap is
usually a .015 to .001 mfd 0603 size surface mount
part.

I've done this mod on many scanners, and the volume
is usually satisfactory afterward.

Good Luck

(All credit for this mod goes to Mr. Parnass!)