The word "reheard" is spelled with the prefix "re-" meaning "again" and "heard" meaning "listened to" or "perceived sound" in the past tense. Its IPA phonetic transcription is /rihɜrd/. The "r" sound in "re-" is followed by a long "e" sound, represented by the letter "e". The "h" in "heard" is silent, but its presence suggests a hard "e" sound in "ear". Therefore, the "e" in "reheard" is pronounced with an "er" sound, represented by the letters "ea".
Heard a second time.
Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874.