I found some great info when I googled canary stopped singing....
http://www.geocities.com/pacificamericansingers/articles/artpc972.htm
Also this.......
I'm going to suggest some changes and additions in the diet. Like most seed eating birds, Canarys need more than just seed. Wild canaries also eat insects, fruits, plant leaves, and get direct sunshine which creates vitamin D. The typical pet canary gets an unlimited supply of just seed. Some get just a poor-quality box mix from the grocery store, which is mostly millet. However, even the best quality seed-only mix is not a complete diet for long-term health. Canary diet should be supplemented with hard-boiled chicken eggs, vegetable greens, calcium from cuttlebone, fruits, vegetables, and greens. You can also offer apples, oranges, bananas, green peppers, canned corn, fresh corn on the cob, cooked broccoli, carrots, raw spinach, raw dandelion greens, raw collard greens, raw Swiss chard, pears, peaches, strawberries, and cherries. The various berries are very good, especially for red factor birds. Iceberg type lettuce is useless and should not be fed but Romaine is fine.
Cooked egg-white is also very beneficial for canaries, and most beneficial during molting.
It's fine to continue giving your bird the seed mix. It's just that he needs some of the other things dailey as well. Obviously you don't try to give him something of all of it, everyday. Another important factor, both in maintaining color and song, but also for the proper absorption of many vitamins and minerals, is proper full spectrum lighting. I'm going to give you a link to information about that. Also, just because you aren't seeing any feathers dropping yet is no guarantee you bird is not about to begin a molt. I'm including a link to information on that as well. While you are trying these things, keep an ever watchful eye out for those illness symptoms I listed for you. Birds are masters at disguising all symptoms until they have become so ill they can no longer keep it up so we have to be ever alert and watchful for the slightest hint that something is awry. If you should see some of those indicators, then it's time to be off to the vet right away.