I think that comprehensible speech consists of vowels and consonants put in some order. Killing one of the two will make something incomprehensible.
I would try to kill consonants to be more exact, this will bring the sample closer to mumbling.
There are multiple ways you could try doing that like fast compressors targeting specific areas and such but fast compression might sound odd (except if you like the compression effect)
One other technique that you could use is to take the channel and try to target the "easy" consonants (Ks Ss Ps Ts) that luckily also introduce the most articulation. The way you can do that is by using a gate and recording the output or by using a plugin like fabfilter de-esser which provides the user with the "targeted" audio isolated and saves you from some work.
Practically you will need to clone the channel you want to mess with (so A is the original and B is the clone) insert the de-essers to B, flip the phase and start tweaking until you start cancelling one consonant after the other.
Keep in mind that if you use a plugin like fab-filter you will need to clone the channel for as many consonants as you want to cancel cause each de-esser will mute the original sound and only keep the part that de-esses.
To perfectly cancel out a sound the amplitude has to be exactly the same so the tweaking part will have 2 phases, one is to target the specific consonant and the second is to fine-tune the volume so it cancels out the consonant of channel A.
EQing is not easy to avoid completely so after all that some eq-ing will be necessary, Group all these channels and EQ the final output. Do not change anything else to the specific channel because if they are not completely identical you will not be able to cancel them.
So a sentence like "The quick brown fox jumps over the lazy dog" will sound something like "The ui brown fo ump oer the lay do" (ymmv)
Haven't tried the above but I have done similar things over the years, it might work for you :)
P.S. I kinda missed the adobe audition part but I think that it takes plugins like any other program