This is an issue with the 1.7.2 client. It may have something to do with the fact that we're currently running an unstable Spigot 1.7.2 version (which technically isn't 1.7; it only allows these clients to connect) while we wait for an official launch from the CraftBukkit team.
I've actually been discussing this quite a bit with other players trying to find a reason why this particular behaviour isn't working properly. As I'm sure many of you are aware, if you switch between 1.7.2 and 1.6.4 your music settings will change. I've had a number of occasions where my particle settings have changed as well (though at the moment I'm unable to confirm whether these settings actively change because I'm not at a computer; I'll check when I'm able too). If you have minimal particles you'll notice that the sound of rain will become muted, so I'm not entirely sure if this setting would also affect the bow impact sound as well.
I'm interested if someone else has been experiencing this and has also attempted experimenting. I'll append the results of my personal tests on this post when I'm able to complete them; hopefully the information will help to diagnose a problem!