The details are in the online manual that nobody reads (but probably should).
WSJT-X 2.3 User Guide (princeton.edu)
In the lower left side of the main page just under the band selection is the receive audio levels (manual section "10.2 Left" but start with "5. Transceiver Setup" - "Receiver Noise Level"). While this is read-only in the software it can easily be controlled in several ways. First (and least easy) is using your computer's audio controls to increase/decrease the input level - typically "Microphone (USB Audio CODEC ). The easier way is to use the 991's RF Gain control (center knob on the right). Typically the good levels are between 40 and 60. Too low you may not get enough signal, too high and you'll overload the input and cause distortion (bands of dotted lines across the waterfall).
For the transmit audio levels (aka ALC on the 991) there are also several ways to control this level. First (and least easy) is using your computer's audio controls to increase/decrease the output level - typically "Speakers (USB Audio CODEC ). Next is to use the 991's DT Gain setting (generally this should be fairly low, not the default 50 or so - mine is set to 20). This should be set to allow you to provide enough room to increase or decrease the ALC using the software's Pwr control (manual section 5. Transceiver Setup - Transmitter Audio Level). Adjusting the Pwr control should change your ALC level on the 991, which in turn changes the transmitter's power output. You want it set about mid way in the blue area of the meter or just high enough that the transmitter power stops increasing. Like the receive audio, too low you won't be heard and too high you won't be copied due to distortion.
Please note that both controls have multiple ways to change settings. It's important that you adjust ALL but think of it as "First" being the rough control (just enough so the later ones change things), with "Next" being the fine control. For the transmit audio levels, the Pwr control is the main "fine" control. Be aware that these "fine" controls may change due to band conditions and especially the Pwr control may be set just fine on one band (or at one time on a band) but be so low that no transmit audio is being sent when you change band or a bit later on the same band that worked fine just a few mins earlier.
