I bought a LG portable super multi drive to play my dvd's, it plays them fine but i get no sound from the acer notebook . When i put my cd's in it the sound comes out fine from the notebook. Even clips on utube will play and have sound.
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Take a look at your sound properties and ensure that nothing is on mute, i.e. wav sound, etc... Also check to see if the volume is turned up on the DVD program itself. If those aren't the issues, you may be lacking the proper audio codecs.
Does the external player have an audio out? If so, plug in headphones or speakers to it and check for sound that way.
Try a different DVD player software, if you are not using it already, try VLC Media Player.