Are you using Push to talk or voice activiated?
You should try Pust to talk 1st.
As it's easier to setup.
Have you verified you mic works?
Are you using a headset with a mic or a desktop microphone?
Hopefully something here will help you out.
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We'll start going down the list of possible fixes.
One possible fix is to try using "Direct Sound"
If you're not already

On the same screen
What do you have for the options under
"Harware Input Mixer (optional )"
Does the box MIXER list a soundcard or does it say noneYour problem may of already been fixed by changing your input device and setting up your sound mixer.
If there's still a problem here is what we will try next.
There might be a problem with using the amplifiers built into vent.
So we are going to make sure you have your Microphone Boost on.
Here is how you check
Go to the setup screen of vent
then click on the record input button

This should bring you to this screen.
Where you want to click on the advance button

Which finally takes you here,
Check Mic Boost if there isn't already
After you put a check in Mic Boost
Close the window & the other window you opened.

You should now be back at the Vent Setup Window
The area in green should look somewhat similiar, where mine says Realtek AC97
There amp area in Yellow you want set in the middle like mine.
The Line Volume in green is where you will want to adjust you mic volume while using the test button to check you settings.

I would like to know if you had your mic boost on.
If not, it's probably a good idea to turn it on anyway.
After turning the boost on, you may have to adjust you line volume down a litte ( area in blue ).
Hope this helps