Really need to know which version of MEP you are using, MEP 2013 displays audio slightly differently.
However you can adjust volume of the audio tracks a a whole by using the Mixer (press the M key) as shown at 1 in the image below and aduts the appropriate slider.
For adjusting the volume on individual objects you use the handle circled as at 2.
In MEP 2013 the video and audio may be displayed as one track, In this case you use the handle circled below to change the volume for an individual object.
Alternatively you change the way the video object is displayed by right clicking it and selecting Audio and waveform display, Video/audio on seperate tracks (Ctrl H) and the proceed as above.
Not to forget that if you want to simply eliminate all the audio of a track, no mater how many objects, just mute it using the 'M' icon on the left by the track number. In MEP 2013 first change display to have audio and video on different tracks.