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There's a few things to check.
1. If you've got HDMI connected to your TV, then the sound may come out of the TV instead of the 3.5mm jack. You can force it by opening a terminal, typing sudo raspi-config and then navigating to the advanced options and the audio settings. You may need to type sudo reboot to get it to work (I don't know, I've never tried it).
2. Test with earphones. Make sure you fully push the plug into the socket. If that works, then remove it. It's not a good idea to use it for long like that. If you did hear audio, then get a 4-pin jack plug (the usual 3-pin works as a quick test, but is shorting out the composite video signal, and it's not intended to be used for headphones neither). Then wire it as shown here in clem57 response:
3. Connect it to powered speakers, the output is line level. It needs amplification.