Does Trunks know how to fire a Kamehameha?
Trunks has never fired a Kamehameha in any of the canon versions of the series. He does use the attack in GT and also in the case of the movie as you stated. Essentially, the attacks used by the Hybrid Saiyans are generally based on the attacks used by their respective fathers. Which in Trunks' case, is the Galick Gun and Final Flash and Kamehameha in case of the Son Family. I wouldn't necessarily say Trunks' scream was made up by the dub with regard to the movie as you can see  here.