I started our box of picture and sound cards last year and then set it aside when I realized how few of the Montessori activities we'd be using. I tackled it again this summer, though. I finished them up a while before Second Son was born but didn't get around to actually publishing the post.
The idea is to use pictures from magazines or greeting cards to illustrate the sounds of each letter. I happened to have a huge stack of magazines I was cleaning out and went through them to pick out some good pictures first. The big expensive magazines definitely had better pictures for a project like this. You won't be surprised that some sounds were much easier to complete with six or eight pictures than others.
I found some of the best pictures in seed catalogs. The most expensive sellers had the most beautiful pictures of all. So our box has lots of fruits, vegetables and flowers in it. (I did try to stick with ones the kids might actually know.)
In the end, I had to resort to searching online for pictures for the last few letters. Most of the sites were not incredibly helpful (showing only pictures I had already found on my own), but I did think the Children's Picture Dictionary had some cute pictures. This Online Photographic Dictionary had some nice ones as well. Sometimes, too, if I knew what word I wanted to use, I would just search Wikipedia. (I would copy the picture to a Word document, print it out, then tape it to an index card. The ones I made last summer were pasted with rubber cement, but since that's apparently toxic to pregnant women and their babies, I switched to a glue stick or double-stick tape for the ones I finished up this summer.)
For letters like x, Teaching Montessori in the Home: The Pre-School Years says to choose words that actually have an "eks" sound, rather than the ones where x sounds like z, using pictures of things with x in the middle or end of the word rather than the beginning. I also picked a few things that start with x with a z sound. It seems right to teach that sound for the letter as well, though I'll probably focus on the "eks" sound first.
I have a few more Montessori projects I finished up the night before Second Son was born. Those posts will come eventually.