First, verify that the input/mic level meter adjacent to the microphone icon in Visimeet is moving. If there is activity when you talk, that may indicate that your audio is getting sent into the meeting and the other side will need to check their audio output/speaker settings (reference this FAQ article for more info).

If you are not seeing activity, then you need to check your microphone configuration:

• If you're using an external microphone, make sure it has power and is not muted.
• While in the meeting, make sure that your Visimeet mic/input slider level is not all the way left (off). Also, check the system audio to confirm that the microphone is not muted or turned down at the system level.
• Finally, navigate to Tools | Options | Audio to confirm that the appropriate audio input device  is selected via the Microphone drop-down.