What is the meaning of 声 in ...车市降价声已经是此起彼伏?
"声(Sound)" is metaphorical in this sentence, "车市降价(Automobile market price drop)" cannot actually produce a sound, so it implies that there are potential of market price drop, because we are hearing "此起彼伏(Wave after wave)" of the "sound" that automobile market price is going to drop.

Also to mention, "此起彼伏(Wave after wave)" usually describes something that produce waves/has up and downs, so "声" is required or it doesn't make sense.