The Brief: "The volume on this bus" comment on TikTok means the viewer relates with the message in the video.

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“The volume inside of this bus” or “the volume on this bus” is a comment seen often on TikTok. It means the TikTok video is relatable to the viewer.
“The volume on this bus” is a shortened version of the phrase, “The volume on his bus is astronomical.” That was derived from TikToker @SnarkyMarky’s Feb. 8 video captioned, “POV: you’re on the bus ride back to school after a field trip and the teacher is yelling at you because your class is too loud. In the video, he yelled, “Ladies and gentleman, the volume inside this bus is astronomical.” The video became very popular, accruing over 14 million views and 3 million likes.
@snarkymarky POV: you’re on the bus ride back to school after a field trip and the teacher is yelling at you because your class is too loud #viral
Subsequently, it became a popular trend and phrase on the app.
The phrase has also been popularized on Twitter.
ladies and gentlemen, the volume inside of this bus is ASTRONOMICAL
#BLACK_LIVES_MATTER pic.twitter.com/fcyHwAMcxV
— angeles⁷ 𖧵 #BlackLivesMatter (@jiminsexycutie) May 31, 2020
The volume inside of this bus- pic.twitter.com/ns3w2LAvjm
— OPAL⁷ 夢 (@Opal7Jimin) June 1, 2020