Why my speaker cannot function normally?

No the pre-amp just gives you enough volume to hear it, the main amp takes it higher. However since the sound is normal on earphones. Then it is the headphone connection that is faulty. The socket side, not the headphones themselves. When headphones are placed in a unit, the plug breaks the contact with the internal or external speakers. But when it's removed it should restore the sound to the speakers. But your unit is not doing that, so there is a bad connection on the socket.

1. Set your Echo Dot and external speaker at least three feet apart. When Echo Dot is close to external speakers, it may be more difficult for Alexa to hear the wake word and other requests.
2. Turn your external speaker on.
3. Plug one end of an audio cable into your speaker and the other end into Echo Dot. All audio from Echo Dot now streams through your external speaker.

Turn down volume / input. If unsolved, then uncheck
"listen to this device" under audio input. On Win7:
R-Click on Speaker in taskbar, click 'Recording devices'
& select earphone properties.

Buy some contact cleaner and flush the ear plug, and push the head phone jack in and out a few times, and repeat 2 or 3 times hopefully that will return the sound through the speakers.

Well, you need to find out what happened. Normally, the earphones will cancel the internal speakers, but the function of the speakers should return after you remove the earphones.
What you need to do is go into the control panel and then to the sounds icon and find out what the system is using for its output. It could be that it thinks the sound should be still going to the earphones. If so, change it to speakers.

check whether motherboard has inbuilt speaker or speaker connected externally.
Externally means check the proper connection is given for the speaker.
If speaker is connected externally in proper manner still no beep means problem is in the external spaeker

Dear sir ...
in fact your son is right so please try another speakers from anywhere and if it work so the old speakers needs to replacement .
thank you

It's your external speakers. Try headsets. Honestly, it'll sound 1000% better for both of you.

In the socket where you plug in the external earphone are bending contacts, which also switch of the speaker when you are using an earphone. Try either putting in the jack and wobbling and twisting it, or tapping the corner with your knuckle.

IN EITHER CASE - BE GENTLE

That will knock of the dirt and the switch will work again

I had the same problem with my Acer 5570 and I just fixed it. What I did is to go to Control Panel > Sounds And Audio Devices >Hardware >On Devices click on Reatek High Definition Audio >Click Properties > On Devise Usage choose the option Do not use this devise (disable) > click OK >then plug in your headphones, click on Realtek High Definition Audio properties again and click on Enable Devise **with the headphones plugged in**. It worked for me.