You don't mention make and model of speaker and whether it's active or passive or the source.
If the speaker is passive then you will need an amplifier to connect it to a line level output. If the output is headphone level you would get a very low sound at best and only one channel not both.
If the speaker is active then you could use a left - right splitter and connect it to one channel of the stereo output. Again you won't get stereo.