I'm about to replace the headphone jack on my Galaxy S3 for the second time. Fortunately, it is an easy fix. You can buy the Headphone jack/speaker unit as an OEM piece ($8-$25 usd) on ebay or amazon. you'll have to remove a handful of screws and be careful with the connector, but it's not a challenging part to swap out.
Of Note: the first time around I bought a cheapo replacement unit, which lasted about two months. This time I sprung for a slightly more expensive "OEM" part, so we'll see if that lasts longer.