Step 1. Place a func_door (brush based entity) somewhere outside your map. You could use any existing door, but you may redo/delete it by accident at some part.
Give the door the following variables:
Target (target) : reset_manager
Step 2. Place a multi_manager (point entity) somewhere in your map.
Give the door the following variables:
Name (targetname) : reset_manager
[create this value by clicking off "SmartEdit" and pressing "add"] (roundreset) : 1
Step 3. For the entities you want to reset each round, just change their names to roundreset. If you've already given the entities unique names, just add another variable with that unique name to the multi_manager with a value of 1.
[Optional] Step 4. You might want to use this technique to set some songs start on/off at the start of every round. You can do this by using a trigger_relay, and this step explains how to use it.
Step 4.1. Create a trigger_relay (point entity) in your map.
Give the relay the following variables:
Name (targetname) : turnoffsongs
Target (targer) : songname (change this to the name of your song entity)
Trigger State (triggerstate) : Off (0)
Step 4.2. Add another custom variable to your multi_manager: [create this value by clicking off "SmartEdit" and pressing "add"]
(turnoffsongs) : 1
NOTE! The value "1" in the custom values for the multi_manager indicates that the entity is triggered 1 second after round start. It could be 0, but resetting entities so fast could cause problems, and sometimes the entities might not be triggered. I usually use 0.1 instead of 0.