No speaker sound

Check your center channel. Most dialogue is routed to the center channel in surround.

The best place to start to prove the hardware works is always with the built-in test tones. The Center Channel must be defined.

This can mean only one thing. Your center channel is not working. Either you have a damaged center speaker of you have a damaged center amplifier. You can confirm by first turning down the volume to normal levels and disconnect any one of the speakers and connect it to the center channel output. If you can hear the dialog the center speaker is defective.
Another way to hear the dialog is to change the surround setting to PL II music or just disable surround and chose Stereo mode.

Your center channel speaker is not working. In modern 5.1 movies dialogue only comes from the center channel. Older movies were stereo/mono so the dialogue would come from both the front speakers.
See if the speaker wires are intact and not loose. After that to confirm if the speaker is OK or defective exchange the center speaker with one of the working ones.If the center speaker does not produce sound, you will have to replace it. But if it works, that would mean that your center channel amplifier is not working & for that you will need professional help.
CHECK CABLES FIRST FOR LOOSE CONTACTS.

" Some DVD\'s sound fine whilst others have background sounds but no voices. I do have an old model TV that has mono sound.Would that cause it? "
Hi,
I wrote a short article for someone who had a similar problem a few days ago.
I will edit it for you so that it fits. Here it is.
Your DVD player should have output connectors in the back of it to plug your amplifier or receiver into. If you have a Stereo system (no surround sound speakers, or front center speaker) connect the DVD Audio Cables on the back of the DVD player to the STEREO (or MIXED ) Output Jacks. If it is connected to the Left Front and Right Front jacks of the "Surround Sound" output jacks instead of Stereo(or MIXED) then the dialog will be sent to the CENTER channel output. If you have no center channel speaker you will not hear it. Thus no dialog.
NOTE: In your case for the TV that is mono, here is the solution.
1 Connect a stereo pair of RCA cables (red and white "male" connectors at each end) to the Mixed or Stereo Output of the DVD player (NOT ANY SURROUND OUTPUTS).
2 Purchase an RCA type "Y" connector that has 2 female to a single male connectors on it. Walmart about 3 dollars.
3 Plug the single male plug of the Y connector into the back of the TV set's single mono audio input.
4 Connect the stereo RCA pair from the MIXED output of the DVD player into the Y connector that is now on the back of yourTV.
5 Run your yellow Video output from the DVD into the Back of the Set's yellow input.
That should do it. The sound should be clear and correct with dialog, effects and music.
Hope this helps,
Best,
Mark

Yes, I can help you. Your DVD player should have output connectors in the back of it to plug your amplifier or receiver into. If you have a Stereo system (no surround sound speakers, or front center speaker) connect the DVD Audio Cables on the back of the DVD player to the STEREO Output Jacks. If it is connected to the Left Front and Right Front jacks of the "Surround Sound" output jacks instead of Stereo is that the dialog on is sent to the CENTER channel output. If you have no center channel speaker you will not hear it.
If you have a surround sound system with two speakers in front and two speaker in back. Then you need to make a change in the Receiver Setup (Which may be set up for 3 speakers in front) so that it is set up for 2 speakers in front.
The last thing my simply be that you have a center channel front speaker that is not completely connected because of a wire falling off, etc.
Hope this helps,
Best,
Mark

If you do not have a center channel speaker connected, then in the reciever setup menu for speakers, there should be a selection for center channel that says something like either yes or no. I'm guessing it says yes. Then you need to select "no" so that the circuitry will direct sound specified for the center channel to be reproduced through your front speakers.

first check the settings on the audio menu of the dvd player. make sur it is set to 5.1 (if available) otherwise set it to stereo and your system should decode it.
assuming the second step is okay : as other dvd you play seem to work) it still may be that your unit is decoding stereo inputs just fine and 5.1 is not working properly....
so if adjusting audio settings in dvd play back menu dont work,
check that the cables to the satellite speakers are connected correctly and all plugged in soundly ( voice is high frequency,- coming from satellites - the garbled noises you hear are the base ( low freq) elements of voice which are coming from the bass box ( that the X.1 part of 5.1) and so that is obviously working. ,

This is a 5.1 (6 Channel Technology) You should hear only the voice thro the center speaker. If the source you are playing is stereo, however the software splits the source its difficult to seprate the voice track, if the source is a DTS 5.1, then you should hear the voice only thro the center speaker. eg: when you play a DVD movie which is DTS 5.1 then you should hear only the voice track in the center speaker, if the dvd movie is just stereo then you will not hear any audio in the center speaker, while the other four speakers get output, this is the special effect in 5.1 ( 6 channel Output).

The problem is that your center channel is not functioning correctly.  Dialogue is typically routed to the center channel only in 5.1, so if you don't have a working center channel all you will get is sound FX and background sound.
Go through all your settings, verify that they are correct.  Verify that your center channel is working properly, and wired to the correct output.
Test the center channel speaker and I feel confident you will find something amiss.