To reproduce:
1. Open attached project;
2. Open "testscene" scene;
3. Enter Play mode;
4. You will hear two sounds:
-"dlg_graves_intro_01 - Dialogue2D" (priority is 0),
-"wall_crumble - Effects"(priority is 64);
Note: The third sound "wall_hammer - Interface" has the lowest priority: 128, so it is virtualized (Max Real Voices in the Audio Manager are set to 2 (Edit > Project Settings > Audio).
5. Open the Profiler window (Window > Profiler or Ctrl + 7);
6. In the Profiler window, click on Audio > Channels:
-Notice wall_hammer sound is virtualized.
7. Open Audio Mixer (Window > Audio Mixer or Ctrl +8);
8. Select GlobalVolumeControl, press "Edit in play mode", and increase the "Master" volume to 1 (or higher);
9. In the Profiler window, notice that "dlg_graves_intro_01 - Dialogue2D" is virtualized, "wall_hammer - Interface" becomes audible.

Result: a 2D sound with audibility above 0 becomes virtualized despite the fact, that it has the highest priority and audibility.

Reproduced on versions: 5.5.4p1, 5.6.2p1, 5.6.2p2, 2017.1.0f3, 2017.2.0b2.

Fixed on: 2018.1.0b2