Each deck has a volume property that you can set to a positive integer number.
0 is mute and I'm not sure if 255 was the maximum loudness or the point where the default 100% marker is set in SAM.
Basically you can set the volume at once (abruptly) like this:
DeckA.Volume := 255;
or fade the volume slider up-/downwards like this:
DeckA.FadeToVolume(255, 3); // Fade from what it's currently set to max and reach 255 volume after 3 seconds
Obviously you can replace DeckA by whatever you like (ActivePlayer, IdlePlayer, QueuedPlayer, DeckB, Aux1, Aux2, Aux3, SoundFX, VoiceFX) and you could also just store the current volume in a variable and then reset it to that after you finished playing your relay.