The best ATC following channel in YouTube is VAS Aviation. You can hear the entire exchange between the CRJ, the Blackhawk and the Reagan tower:

YouTube: Audio of MID-AIR CRASH into Potomac River | Regional Jet and Black Hawk Helicopter (VASAviation -)

The callsign for the American Airlines flight was Bluestreak 5342 and the Blackhawk's PAT25. DCA Tower asks the AA flight to change to runway 33 as they fly over the bridge and 5342 acks the instruction. The American flight was following normal protocol for an approach in to Reagan.

Also in the area was the Blackhawk, flying up the river. At the 1:33 mark the tower tells the Blackhawk to look for the CRJ (Which is approaching runway 33) and notes its altitude. The tower approves "visual separation", which is an instruction for the Blackhawk to visually look for the CRJ and maintain safe distance.

At 2:17 the tower asks PAT25 if he's got the CRJ (Bluestreak 5342) in sight, then gives instructions for PAT25 to pass behind it. There's an inaudible portion, then the tower says "Vis sep approved". I don't know if PAT25 told the tower they had the CRJ in sight or not.

After that you can hear gasps in the tower and another flight (AA3130) asks the tower if they just saw what happened. The tower controllers then move to divert all other incoming traffic and close Reagan down. AARF (fire and rescue) requests access for all taxi and runways so they can roll trucks to the scene.

That's some of the most congested airspace in the country; I have no idea why they let helicopters fly up the Potomac straight across the glidepath of active runways. There's traffic literally turning in right there...that seems insane to me.