In the cruise, you are normally headset off, loudspeaker 1 o clock’ position. So in case of emergency descent, you first put the mask, then your headset. The other way around might be difficult and uncomfortable especially if you were big headset, you won’t be able to put the oxy mask on top of it. You can experiment it. We don’t rely on loudspeakers during emergency descent. We do a quick « intercom check » then intercom back to off to avoid the noise created while you breath in the oxy mask. Only turn the intercom On when you need to talk.
We always set loudspeaker to minimum in my company while using headset as otherwise it creates an annoying echo. We are using the TELEX 850.