Based on your query, I recommend using Project Read. This AI-powered platform is designed to provide personalized reading support for students, making it an excellent tool for learning letter sounds.

The key feature that will be most beneficial to you is the AI-Powered Tutor. This feature listens to each student as they read and provides live feedback based on their mistakes at the phoneme level. This means that it can help students correct their pronunciation of individual letters and sounds in real-time, which is exactly what you're looking for.

Additionally, Project Read generates Personalized Decodable Stories for each child according to a K-2 phonics scope and sequence. This feature helps students learn reading through decodable stories that contain only those words with patterns that they have been explicitly introduced to. This means that students can practice their letter sounds in the context of real words and stories, which can help reinforce their learning.

Finally, the Live Feedback and Encouragement feature provides students with immediate corrections and positive reinforcement as they read aloud. This can help students feel more confident and motivated to continue practicing their letter sounds.

While Project Read is not explicitly stated as free, it is designed to be cost-effective, with individual components sold separately. This means that you may be able to use some features of the platform for free or at a low cost.