If you're having an issue with your voicemail audio not playing there could be a few settings that could be causing this issue.
Firstly, Lets make sure that the volume on either the phone or computer you're using is actually turned up. It might be a simple step but it can be easily overlooked and could save you time.
For Android devices :
If the voicemail audio is being played but you're not hearing anything, either while the voicemail is playing or once it's stopped, attempt to increase the volume by turning it up with the volume up button located on the side of your phone. 
Once you do this, you should see something exactly like the screenshot above, you can pull down with the arrow to the right, and then you can check to see if the media volume is all the way up. If not, please adjust. 
Sometimes you can't hear because the media volume is low, please double check that to see if that's the case. In the event that this doesn't resolve the issue, then you will need to check the output of the audio. The audio can be played from the bottom speakers or the receiver (where you place your ear).
The option displayed above will have the audio played through the receiver (where you place your ear). 
The option displayed above will have the audio played through the speaker, ensure that the volume is turned up.
The option displayed above will have the audio played through the bluetooth receiver or the headphones you have plugged in. Disconnecting / unplugging the headset will revert to the two default audio options 1) Speaker 2) Receiver.
For iOS devices :
Ensure that under "Ringer and Alerts" that change with buttons is disabled, having it enabled will result in having to use the volume buttons while simultaneously listening to the desired audio. If you try to do it without playing audio, it's the phone's ringer that is adjusted and not audio volume. 
Once you've confirmed that the audio is indeed turned up, please double check to see what the audio output is. This is only available for iOS devices. 
From within the settings app, you'll navigate to YouMail and ensure that the audio output is set to "Speaker"
In the event that you've tried all the above and you're still having audio issues, one last thing you can try is uninstalling and reinstalling the app. This should resolve the issue. If not, then you'll need to reach out to us so that we can further investigate the issue. You can start a chat or submit a ticket and one of our agents will respond as soon as possible.