connect the Ground of the Arduino to the 'sleeve' contact of the mini phone plug that normally goes it the PC's headphone jack. The sleeve is the long contact at the base of the connector. Connect the output pin to the 'tip' and 'ring' contacts of the mini phone plug. The 'tip' is the end of the plug and the 'ring' is between the 'tip' and 'sleeve'. They are the left and right audio channels.
You will need a USB Host adapter and special software to get your Arduino UNO to read from a 'pen drive'.
Why are you talking about a STEREO PLUG when
an arduino doesn't have STEREO ?
As stated , you need a STEREO amplifier and software that outputs left and right channel audio on separate pins with capacitors in series to block the dc.
Hi,
What is the impedance of the speakers?
What resistance do you measure across the "speaker" terminals/wires you will be connecting the Arduino to.
OR
Are they speaker/amplifier combination?
Can you please post a picture of your "speakers" including any leads/connections it has.
If your "speaker" has impedance of 8 Ohms then connecting it directly to a digital output, 5V logic.
Do not confuse "impedance" with "resistance". The impedance is a measure of the effective AC resistance at some frequency, perhaps 1000Hz. The DC resistance will be a fraction of 1 Ohm.
Paul