You can set a normal sound play to NONE in object editor and use a custom sound by trigger when player selects a unit.

Events


Player - Player 1 (Red) Selects a unit

Conditions

Actions


Sound - Play BeastiaryWhat1 <gen> at 100.00% volume, attached to (unit)
You can also to play different custom sounds to be played with some chance randomly (random number function).

Actions
The plus of this way that you can make some sounds to be played more often, other more rare.
But, all sounds has a large size.