Learning the associations between letters and speech sounds is a crucial step in acquiring reading skills. Previous studies have found that the brain's process of linking letters and speech sounds is highly dynamic during initial learning and takes years to become fully automatic. However, learning-related changes (e.g., newly-learned and overlearned) in this audiovisual processing are not well characterized. In this study, we first trained adult native Finnish speakers to associate novel foreign letters with familiar Finnish speech sounds. We then directly compared the audiovisual processing of the newly-learned (one day after learning the foreign letter-sound associations) and the overlearned Finnish letter-speech sound associations using magnetoencephalography (MEG). We observed different visual processing patterns for the two types of letters, with larger responses for the newly-learned letters than for the overlearned letters. The audiovisual interaction (A+V-AV) also showed different patterns in the left hemisphere. Most interestingly, the audiovisual congruency effect (audiovisual congruent vs. audiovisual incongruent) was found for both responses to newly-learned and overlearned stimuli in the left hemisphere. The congruency effect appears to be stronger and processed faster for the overlearned stimuli than for the newly-learned audiovisual associations. The results indicate different brain processes for newly-learned compared to overlearned audiovisual associations, which likely relates to the different levels of automaticity in audiovisual integration.