Got this idea from another user who uploaded a Family Guy fishing sound bite. IMHO this one is much better.
This wav file will replace the default bobber splash sound with the classic Bert and Ernie Sesame Street sketch where Ernie fishes by calling the fish with a fish call. "Heeeeeeere Fishy Fishy Fishy"
FishBite.wav belongs in your WoW directory of \Data\Sound\Spells\TradeSkills
You will most likely have to create the above folders if they are not listed in your default WoW install directory.