Tapping on the volume button will bring up a volume control overlay on the lower part of the screen. It uses the nomenclature of 'decibels' to match the Foobar2000 volume control.
To make the volume overlay disappear, you can either tap the dedicated Windows Phone back button ( <-- ) or you can also click on the volume button again.
For quick accessibility, the volume control is available on the 'now playing', 'search', 'playlists', 'queued tracks', and 'favorites log' screens.
'0 dB' is max volume and -100db is mute.
dB is a logarithmic scale, so changes are not linear.
Also note that this control sets volume only in Foobar2000, which is subordinate to the master volume settings of your PC. So if the Foobar2000 volume is set to max, but your PC volume is muted, you won't hear anything.
Tapping on the up arrow button will increase volume, while tapping on the down arrow button will decrease it.
Use short, quick taps on those buttons to go up or down on volume. Tapping and holding will not auto repeat.
If you are between 0 to -50dB, the up, down arrow buttons, increment or decrement by 3dB with each tap. Between -50 to -100dB, that changes to 6dB since even -50dB can be essentially 'mute'.
Tapping on the Max volume button (top right speaker icon) sets volume to maximum (0dB).
Tapping on the Mute button (bottom right speaker icon with X), mutes it by setting volume to -100dB.
The mute button acts as a toggle to essentially mute/unmute.
If not on mute, tapping it will mute. If on mute, tapping it again will restore volume to whatever you had it set to before the mute.