If your DVD player starts to slow down just a few minutes into the movie and the audio becomes so low you can just barely hear it, if at all, try this troubleshooting step. Shut down the DVD player and unplug it from the television (or cable/satellite receiver). Disconnect the DVD player's power connection as well. Leave the DVD player completely unplugged in this manner for at least thirty seconds. Plug it back into the television and then plug it into the wall. Turn the DVD player back on. If this fixes the problem, it may have been a "soft code" issue. Disconnecting and reconnecting the DVD player will reset these codes and may resolve the issue. If it happens again, or if this fix does not work at all, it is time to buy a new DVD player. The repair for this type of problem would probably cost more than a new machine.
Incidentally, power cycling a DVD player in this manner can fix a number of quirky problems. Try it before attempting more difficult troubleshooting and repairs.