I have a tiny FM radio(made by Philips). It has only a 3.5 mm jack and no speaker. It works well with a headphone. I try to connect it to my PC speakers but it does not works. what should be done to connect it to an external speaker? please help.
Small headsets are high impedance output devices probably more than 32Ω and often more than 120Ω. For max power coupling the amplifier wants to match that impedance. Max power coupling is pretty important becasue for headsets we are talking milliwatts of power.
Your Computer speakers might be 8Ω. Because the FM radio is High Impedance most of its potential output power will be lost internally trying to drive the low impedance load.
If your computer speakers are amplified they might also have a threshold problem. They could be biased to damp out low signal levels in order to prevent hissing noises, and unintended radio detection problems.