At the top of the screen are the Vu-Meters:

There are as many meters as there are channels.

Each meter shows the signal of every channel. Most of it will be square waves, but you can also see triangle/sawtooth waves if hardware sounds are used.

There are 16 steps, from 0 to 15, 0 being "mute", 15 being "full volume". On the left you may see two small vertical bars:

• The left bar is the current volume generated by the PSG. Obviously, it is not as accurate as the wave behind, but it's main purpose is to show if a sound is hardware or not: when it is hardware, the bar becomes red, indicating that the wave generated and displayed behind will probably not be a simple square wave.
• Besides is another blue bar showing the noise, from 1 to 31 (1 being a high-pitch noise, 31 a low-pitch noise). The bar is invisible if there is no noise (like in the image example).

Also, note that the scale is logarithmic, as the volume curves of the AY/YM are also logarithmic: the difference between volume 14 and 15 is much noticeable than between volume 0 or 1.

 

You can click on any meter to mute/unmute channels:

Left-click Mute/unmute the channel under the cursor.

Right-click Mute all the other channels, unmute the clicked one. Another right-click will unmute all the other channels.