Be it bad ears or a pretty rudimentary testing method, but the experience we had at a lan was that we could hear each other through mumble before hearing a shout from across a room.
I am on my way to bed but, damn, you're getting me curious about the delay that Mumble has versus speed of sound. Knowing it uses Speex ("This is an example of Speex ..." x100) which has a minimum delay of 30ms (https://en.wikipedia.org/wiki/Speex), and that you have <1ms ping on gigabit LAN so ignoring that by comparison (even if you>mumble>someoneelse is 2 hops), sound travels...
$ qalc
> 340m/s * 30ms
(340 * (meter / second)) * (30 * millisecond) = 10.54 m
If you say it was ~11+ meters (36 ft), I may be curious enough to create a test setup :p. But I'm assuming you mean the person sitting next to you, not someone who put Mumble on huge speakers across a hall.
It was also in 2009 so it could even have been Celt rather than speed or opus