Your notebook plays MP3s and music CDs correctly and at full volume. But for videos and DVDs, the volume seems to be a little lower than usual. This unsatisfactory and frustrating situation is caused by a faulty AC3 driver implementation. The fact that the audio is even audible means that the AC3 driver is working somewhat, but apparently a setting for the downmix to stereo output on the notebook is not configured correctly.
What can help you is a small tool called AC3Filter. It allows media players to playback movies with AC3 and DTS audio tracks. It is possible to upmix any audio source to 6 channels. Filter can do multi-channel and digital (SPDIF) output. It can encode any audio source to AC3 on-the-fly and send it over SPDIF to the receiver.
Usually, just installing it should do the trick, but if the sound is still not clearly audible after that, go to the sound settings in the Control Panel and select the setting ‘2/0-stereo’ and under ‘Preset’, select ‘Loud’. After restarting your computer, the playback of AC3 DVD audio will work perfectly.