New Solo Design and Features

In Ardour 2.x the solo system had complete separation between track and buss soloing. Any track that was solo'd would mute other tracks but not busses and visa versa.

Ardour 3.x introduces a more cohesive structure that not only links busses and tracks but also makes sure that the path to or from the solo'd track or buss is solo'd as well. As an example soloing a buss will also solo any tracks feeding it, and any busses it may feed into.

Two additional controls on each track and buss further fine tune how solo/muting works. These are Solo Isolate and Solo Lock.

per track solo controls

Solo Isolate

Solo Isolate, as the name suggests, isolates tracks or busses from the solo system. When tracks or busses are solo'd the isolated ones will not mute.

Solo Lock

Solo Lock locks the solo into its current state (ie solo on or solo off). It will not allow the solo state to be changed until the lock is released.

New Solo Controls

If you use the new monitor section then AFL (After Fader Listen) or PFL (Pre Fader Listen) provide a non-destructive solo. They allow monitoring of a signal at any position in the audio path that an AFL/PFL is provided, and does not interrupt the audio to the master out - the effect is heard only via the monitor path. The choice between Solo and AFL/PFL is made in the monitor section of the mixer window, which also shows the current solo/isolated/audition status:

monitor section solo controls

There are also new controls for:

Solo Boost
provides extra gain for soloed tracks. Useful when soloing material where all or most tracks are particularly quiet
Solo-In-Place Cut
Controls the gain reduction applied to un-solo'd tracks and busses when soloing. By default, -∞ dBFS (so unsoloed tracks are inaudible). Decreasing the level of cut makes "Solo-in-Front" possible, where unsolo'd tracks/busses merely become quieter and solo'd tracks/busses "stand out in front".
Exclusive
This toggle control affects whether or not soloing one track or bus cancels any existing solo
Solo/Mute
This option was present in Ardour 2.X (hidden in the Options menu) and controls whether or not Solo overrides Mute (i.e. is a solo and muted track audible or not?)
(See the monitor section page for info on the other controls visible in this shot).