I've never used Audition before, but need to know how to muffle dialogue (an argument with fairly loud voices, recorded in normal conditions) to make it sound like it's coming from a different room on the floor below.
I figure I would just have to play around with the low and low-mid frequencies in the EQ, but I'm not even sure how to change the EQ in Audition, so any help will be appreciated.
It would also be helpful if someone could tell me how to make the EQ gradually go back to 'normal' over a period of a 4 or 5 seconds.
There are specific 3rd party plug ins that do the job perfectly (Audioease do one called "Speakerphone" which is excellent). The way to do what you're talking about would require an EQ and the ability to 'automate' the values of the high frequencies.
Think of the way that walls and floors behave, if you were to remove the low frequencies as you're suggesting you would be left with a really thin sound in the voices which is nothing like the effect you want. Walls and insulation will remove and muffle the top-end of the frequency scale leaving the lower end to be heard so that's what you're trying to recreate. Using the EQ option at the left hand side of Audition should bring you up the options. Take the slider or handle at the FAR RIGHT of the freq. chart and drag it down to about 200Hz, this will remove the top end and take the emphasis away from the voice which sits around 4Khz in the top-mid range.
As for making the values change over a period of time, I don't know how to do this in Audition as I don't think Audition allows for automation of values (although I could be wrong!). In a sequencer such as Cubase, you need to activate the automation track and then choose which parameter you want to control over time, apply a curve or slope so that it the values change according to what you want.