Like this
This is just one situation that comes to mind. Picture two guards standing at their post:
[Player] I cast Ghost Sound around the corner behind them, forming the sound of 4 men charging.
[Guard1] "Did you hear that?"
[Guard2] "What was that?"
[DM] Rolls saving throws for the guards because they are "interacting with it".
[Guard1 fails] "Egads! We're being overrun by soldiers! Retreat!"
[Guard2 saves] "Wait! I don't think it's real. Come back!"
Picture two guards standing at their post with orders not to move:
[Player] I cast Ghost Sound around the corner behind them, forming the sound of 4 men charging.
[Guard1] "Egads! We're being overrun by soldiers! Retreat!"
[Guard2] "We can't leave our post! Let's hope reinforcements arrive in time! Prepare for an attack!"
They didn't "interact" with it, so they automatically believe.