Ref headsets vs speakers and hand held mics...
JAR-OPS 1.313 Use of headset
(a) Each flight crew member required to be on flight deck duty shall wear the headset with boom microphone or equivalent required by JAR-OPS 1.650(p) and/or 1.652(s) and use it as the primary device to listen to the voice communications with Air Traffic Services:
(1) on the ground:
(i) when receiving the ATC departure clearance via voice communication,
(ii) when engines are running,
(2) in flight below transition altitude or 10,000 feet, which ever is higher, and
(3) whenever deemed necessary by the commander.
(b) In the conditions of paragraph (a) above, the boom microphone or equivalent shall be in a position which permits its use for two-way radio
communications.
Edit to say we use the intercom all the time for chatting as required by our ops manual along with ANR headsets.