The important thing is that he is eating, as that probably means he isnt feeling too bad.
Here are the things that come to mind regarding his loss of voice:
This is most likely to be inflammation around his larynx/pharynx (laryngitis/pharyngitis).This could simply be an infection - just like you or i getting a sore throat. This is likely to improve, although may need a course of antibiotics and antiinflammatories from your vet if it continues.
There is a possibility of something being stuck in his throat, although the fact that he is eating suggests this is unlikely. they tend to gag constantly and sometimes salivate a lot.
If he has previously vomited, then sometimes this can 'scald' the back of the throat and lead to pharyngitis.
I think if he is eating well, then a pharyngitis is most likely, so hopefully things should improve. The usual applies though - if he is unwell in himself, or if it continues, you should see your vet.
I hope this helps. Do let me know how he gets on.