I found that 0.00011 is the lowest volume it can be to still tick over. Tested it using the by binding to OnAudioPlaybackPercent and printing the playtime percent. As to why that is a special number, I have no idea, it beats me, found the number used for a few different things in the engine that is audio related. Potentially this?
float SmallSize = FMath::Max( SmallDecay * 0.05f, 0.001f ); // 0.0001->0.05, 0.0048
within
void FCoreAudioEffectsManager::SetReverbEffectParameters(const FAudioEffectParameters& InEffectParameters)
in CoreAudioEffects.cpp
(I was doing some PS5 Controller Speaker work for more context, so may not be true in all cases)