Starting with the most common problem with a new Mepis install, try this first:
1] in the system tray in the lower right of the screen, right-click on the sound icon and chose "show mixer window."
2] When it appears, you may need to click the button at the bottom "Mixer" What you want is the full panel of sound mixer options, as shown in the picture online:http://wm59.inbox.com/thumbs/6_cee37_f8 ... .png.thumb
2a]Your mixer may differ, as what shows up depends on what's available on your sound card.
2b] You may need to go to Settings > Configure Channels in the menubar of that window to make sure everything shows up in the mixer window.
3] Not only the Master volume slider needs to be up, but also the PCM (blue squiggles icon) channel and possibly the "front" or "main" channel if your sound card has this option.
If that doesn't do it, let us know!