Associating letters with speech sounds is essential for reading skill acquisition. In the current study, we aimed at determining the effects of different types of visual material and temporal synchrony on the integration of letters and speech sounds. To this end, we recorded the mismatch negativity (MMN), an index of automatic change detection in the brain, from literate adults. Subjects were presented with auditory consonant-vowel syllable stimuli together with visual stimuli, which were either written syllables or scrambled pictures of the written syllables. The visual stimuli were presented in half of the blocks synchronously with the auditory stimuli and in the other half 200 ms before the auditory stimuli. The auditory stimuli were consonant, vowel or vowel length changes, or changes in syllable frequency or intensity presented by using the multi-feature paradigm. Changes in the auditory stimuli elicited MMNs in all conditions. MMN amplitudes for the consonant and frequency changes were generally larger for the sounds presented with written syllables than with scrambled syllables. Time delay diminished the MMN amplitude for all deviants. The results suggest that speech sound processing is modulated when the sounds are presented with letters versus non-linguistic visual stimuli, and further, that the integration of letters and speech sounds seems to be dependent on precise temporal alignment. Moreover, the results indicate that with our paradigm, a variety of parameters relevant and irrelevant for reading can be tested within one experiment.