First, determine if their audio is actually reaching you. When they talk, is the level meter to the right of the speaker icon in Visimeet moving? If you are meeting with more than one other person/site, are you able to hear some but not others? If answer to either of these questions is yes, then the other side is probably not sending audio. They should likewise reference this FAQ article.

If you are receiving audio (e.g., seeing audio output meter activity yet not hearing any of the other participants), then you need to check your speaker configuration:

• If you're using external speakers, make sure they're turned on and the volume turned up.
• While in the meeting, make sure that all Visimeet volume sliders are minimally set to mid-range. Also, check the system audio levels confirm system audio is not muted.
• Finally, navigate to Tools | Options | Audio to confirm that the appropriate audio output device  is selected via the Speakers drop-down.