noun
second audio channel; supplementary sound channel
Refers to an additional audio track, often used in broadcasting for commentary, alternative language, or descriptive audio.
この番組は副音声で英語の解説を聞くことができる。
You can listen to English commentary on the second audio channel for this program.
主音声 is the main audio channel, while 副音声 is the supplementary one.
Compound of 副 (supplementary) and 音声 (sound/audio).