Function Button issues

Try VLC media player. Or a different one than what you have now

click start control panel sounds and audio then select audio make sure the default player is the media you want to use as your default sounds also
click start control panel sounds and audio devices select advanced there might be a tick in the mute box untick it
hope this helps

Just click over a mp3 file with the right button of your mouse and select open with on the menu.
And then click to open on Windows Media Player.

If audio is choppy it indicates the processor is "busy" and it is buffering due to some other process using up resources and keeping the machine busy so it cannot stream the audio properly. Possibly check the running processes and see which ones have high cpu usage and turn them off, then try it. It may also be a media player issue - have you tried updating the media player or reinstalling it - sometimes installing a service pack will cause a problem with the actual media player - reinstalling them usually fixes that problem.

did u intalled KLC codec pack or any other codec pack, if it is try to reinstall the codec pack and restart the pc and try.

Check if windows media player got the audio codec for him

Download the latest version of the Philips Deivce Manager from Philips website and use the Update option on the new version. This player requires a firmware upgrade to make it compatible with Windows Media Player 11, and the Update function in the Philips Device Manager will perform this for you.

-Tha Mp3 Doctor

May be, the settings of your dvd player is not ok. Try another DVD Player.

Probably faulty headphone jacks/controller, but reset all the defaults in the menu to be sure. Additionally you can try these soft fixes, but they probably don't apply to your situation:
These are the most common methods for recovering Sandisk's Sansa Mp3 Players. I have been almost universally successful in recovering Sansa players using one or a combination of these methods. These methods do not work if you are having synchronization or music format issues - they are for recovering a player that will not power up, freezes, flashes, is not recognized by the computer, and a host of other issues.

Reset the Sansa Player first. On most models, restart is done by holding the power button in for 15 seconds or longer. Check your specific manual to be sure. The manuals are available from the Sansa website if they have been misplaced.
Download the Sansa Recovery Tool from the Sansa website.
Format - Erases Everything: Connect your Sansa player to the computer and open Windows Media Player. Click on the Sync tab. Pull down the drop down menu underneath the Sync tab by moving your cursor over the Sync tab and left clicking on the little black arrow underneath the Sync tab. Highlight "Sansa 1GB" (wording may vary slightly), and then select "Format." - this should erase everything.
Firmware Upgrade- Erases Everything: Get the latest firmware upgrade from Sansa - it updates the firmware and erases every song in the player all at once. Note: this method doesn't work if already have the latest firmware. Go to the Sandisk website and click "Firmware Updater" to find out.
If these fail, then:
Try this: disconnect the player from the computer, turn it off, hold in the volume + button, and reconnect the player to the computer while holding the volume + button in. Keep holding the volume + button for an additional 5-50 seconds. Windows should eventually go into recovery mode and revive the player- it will pop up a window that says Found New Hardware Wizard.
If you get the problem where MTP Device keeps popping up continuously, uninstall the “MTP Device” by going to Start à Control Panel àSystem à Hardware à Device Manager (you computer may vary slightly, depending on Windows Version).
You may have to restart your computer after each of these methods to get them to work properly. Disconnect your Sansa player BEFORE shutting down your computer.
-Tha Mp3 Doctor

try downloading the windows media player 11
(that fixed a lot of my sound problems)
hope this helps