That really depends on the pilot. The sUA pilots will remember it as Ch.9 but I haven't seen that term used internally for a very long time. The switch is labeled "OBS Audio" on the 737 which is very helpful, either.

In general, the awareness of the feature, and particularly the enthusiasm many passengers have for it, is very low in the 737 fleet after all these years of it not being available on them. That's why I recommend asking, as often as possible, to help raise awareness.