An adjustment method of sound output is disclosed. The adjustment method includes the following steps of: receiving an audio message having a vowel message; determining whether the audio message is a whispered voice message; if the audio message is a whispered voice message, outputting a normal voice message, wherein the spoken content of the normal voice message is the same as that of the audio message, and the normal voice message has a normal voice vowel message, wherein the sound energy of the low-frequency part of the normal voice vowel message is 1.5-1,000,000 times that of the vowel message.