Pressing volume keys causes songs to skip
This is a feature of CyanogenMod. Pressing and briefly holding one of the volume buttons when the screen is off will cause the music player to skip forward (volume up) or backward (volume down). The intention is that this allows you to skip tracks and whatnot without having to unlock the device.

If you find it annoying then you can disable it completely in your settings: simply uncheck the option at Settings -&gt; Sound -&gt; Volume rocker music controls. Alternatively, try to use short presses of the volume buttons when the screen is off and it should adjust the volume rather than skipping.