Can't turn off internal speakers when i have external speakers connected via headphone socket

Not directly.
You can:
play it via AUDACITY on your computer, or
amplify it from the line-out RCA sockets*, via your sound-card line in or a separate hi-fi amplifier with equalizer, or
use a headphone-splitter cable and amplify from that to powered speakers.
*The downside of RCA sockets is that your player's internal speakers still work.
Anything plugged into the headphone socket cuts out internal speaker. The headphone splitter allows monitoring via a headphone aaas well.

It will be so. When you insert the headphone pin into its socket, the internal speaker connection will be automatically cut off, and you can listen the sound via the headphone only.

Hello
You are connecting external speaker from your TVs headphone socket I thinks. It is quite normal that the internal speker will cut off when external a headphone is connected to the headphone socket. OK.

Generally when the head phone jack is inserted the speakers will get disconnected automatically.

If you are going to use external amplifier you can try the AV sockets. Opening the TV you may require the special screw drivers some times.

once you connected with the external speakers the internal speaker will never work

From the spec sheet I was able to find it APPEARS that the only way to connect speakers to this unit is via the headphone jack. There seems to be ONLY input audio, no output other than the headphone jack.

Using this jack will likely turn off the internal speakers and you will have to use AMPLIIFIED or "ACTIVE" speakers if you want external ones.

If you connect external devices to this, make sure they are all fed from the same receptacle to avoid ground loops and possiblle damage.

Hi,
    This problem arises due to cross pin connection of the headphone socket.Mostly frequent use of the headphones result this situation (no sound via external speakers). Call any service man he will provide you the services at nominal charges. And if you can do it by yourself then approach to the headphone section and just adjust the headphone pin, This will solve the problem.
Thanks for using fixya.

That is not a software problem.
The headphones/external speakers jack socket has a switch to exclude from audio the internal speakers, when a jack plug is connected.
This is a very simple device and is not software controlled.
The fact that it has gone faulty at the same time you reinstalled the system is a coincidence.
If it does not mute when you insert headphones, all you can do is trying to play with the jack plug , inserting and pulling it out. If this does not unlock the switch, the alternative is having the jack socket replaced by a technician. This is generally an expensive repair, asthe socked is soldered on motherboard.
A cheap alternative to a proper repair is using an external USB to Audio Adapter and mute the internal audicard when using headphones.

The internal speakers should automatically be disconnected by inserting a 3.5mm jack plug in the audio out socket. It may be that your headphones or external speakers are using a 2.5mm jack which is too small to operate the switches in the audio out socket. Purchase a 3.5mm jack plug [cheap] and plug this in if it disconnects the laptops internal speakers then you need to connect accesories with the correct size plug.