Unfortunately the event can't know which round is the final round in which rebuys are allowed (currently), so you'll have to update the event if you decide to change the last round for rebuys.
Create a new event (from scratch). Select the trigger "A level starts". Set the Conditions to:
roundNum = 5
This will make the event fire at the start of round 5. Change "5" to your last round for rebuys. Add an action that plays your audio file. Add another action that displays a message. Press the OK button to create the event.
Create another event from scratch. Select the trigger "The clock ticks". Set the Conditions to:
roundNum = 5 and secondsLeft = 300
This will make the event fire when there are 300 seconds (5 minutes) remaining in round 5. Again, change "5" to your last round for rebuys. Add the same two actions as above.