I suggest you pre-record the video in this case. And record your commentary/inner monologue afterwards. That would give you some time to think of good stuff to say instead of going "oooh shiny, a quest marker" all the time. Though, if you'd manage to pull THAT off as a running gag you'd have earned my respect.
On another Note: Using an echo effect just because it's an inner monologue probably isn't a good idea, especially if you do it all the time. Sometimes it's too hard to understand. If you still wanna, though:
REAPER is a mixing program. It's not freeware, but it's an unlimited, fully functional, trial and it has a nice effect library you could use
Also:
This thingy isn't an LP but does the in-character monologue for Half Life.