Why can I see the presentation but not hear the audio?
It sounds like you were connected to the session successfully, but your audio is not yet connected.
You should have either Phone Mode or Computer Mode selected in your audio settings, which will connect you to the audio conference.
If you're using the desktop app, take a look at the Audio tab in the Options pane of your Control Panel. See Switch Audio Modes in Session for more details.
If you're using a mobile app, open the Audio settings (either under the Audio tab or Settings tab).
If you're using the Web App, click the Setting icon in the toolbar and check the Audio tab.
If that didn't resolve your issue, also see Why can't I hear anyone?.