where you plug in speakers/headphones
When you plug the headphones in, that disconnects the speakers, and no sound will come from them. Unplug the headphones to hear the speakers.
Check your settings.
You plug in your headphones and turn up their volume as well as your speakers volume.
Many amplifiers automatically disconnect the speakers when a headphone plug is inserted, unplug the headphones and the speakers should come back on.
That depends on whether or not your computer has speakers. If it does, then unplug your headphones, plug in the speakers, and make sure your volume is turned up. If you don't have speakers, then there is no way to do have sound without headphones other than to buy speakers. Most desktops have speakers built in to them, though.
Plug headphones in.
Nope - if your sound is coming from both the headphones and the built-in speakers, it's your headphone socket in the laptop - not the plug that's at fault ! Basically an audio socket is a switch. While no external speakers (ie headphones) are connected, its internal components direct the sound to the computer's built-in speakers. Inserting a plug into the socket breaks the connection to the internal speaker circuitry, and directs the sound to the headphones.
lower the volume or plug in some headphones
The same way any set of speakers works - by sending an analog audio signal to an amplifier connected to the speakers. Whether the speakers are integrated into the monitor or stand-alone units, the wire from the speakers/monitor must be connected to the audio output of the computer. This wire terminates in a 3 conductor mini-phone plug. The jacket of this plug and the proper jack on the rear of the computer are usually light green in color. Under the jack, the mounting panel will have 'SPKR' etched into the metal/
How about getting a suitable adapter cable and plugging the other end into a hi fi system and then use headphones and speakers. Alternatively you can try plugging in the headphones halfway. It works apparently.
Generally speakers are plug-and-play. If you have tried that and it didn't work go to your control panel to 'Sounds and Audio Devices' to 'Speaker settings' then 'Advanced' and change the scroll bar to 'Headphones'