Several options. Some folks have recommended a Blue Tooth speaker, works nice if your computer supports BT.
I have a small "FM Transmitter" here, plugs into the headphone jack and feeds the radio.. Most computer stores including Radio Shack carry one or another version of it. Set of amplified computer speakers.. Same source list..
In days of old when folks were bold one of the accessories for a computer I sold (Worked in a store that sold that is) was a very nice audio system,, but then todays computers are not that good.