Well sound travels better in water than air. I am not sure if fish hear the music or feel the vibrations it creates. From the papers I have read on schooling it would appear that fish school based on what the fish closest to them in a gtoup are doing. As for mate selection, I would argue that nature has ordained that the top fish spawn with other top fish. The goal is to produce the healthiest and strongest offspring as this gives both the individual and the species the best chances at survival.
As for avoiding low oxygen area i would say this is a corolary of #1 above. There are other threats besides being eaten fish must avoid.
I have a theory about the runts in almost any group of fish. We know that fish will compete for food. The bigger strobger fish will always get to eat first. It necessary they will fight off other fish to be able to get the food first. So there will be a few fish which cannot hold their own in a fight for food and will get beaten up or even killed if they challenge the bigger fish which are higher ranked within the group. Such fisjh will develop a strategy of not challenging for food, They accept less or even none on occasion as a way to avoid being beaten up every time they try to eat. This results n their being smaller in size than most of the group.
Their smaller size makes them less of a threat to the bigger fish in the group and this means less chance of their being attacked/ essentially, such fish are choosing to make themselves smaller by not fighting for their share of food which means they get less. We often see sch fish hanging away from the aggression we see at feeding time. They will accept any smaller scraps that may become available as they drift away from the main ball of fish and food. I think this is more common in aquariums than in nature. Basically, being a runt can have its advantages.
I noticed this sort of behavior in my Ancistrus grow-out spaces. As the fish in such set-ups got bigger, I would start selling them. And\g as I began removing the biggest fish from a group what I saw was the runts becoming more aggressive in their eating. Most of them would then put on a growth spurt. They would eventually get to the same size as the other fish, they just did so at a different rate.
I have adopted my own way of delivering food. First, I try to keep it spread out. For example, when I feed Repashy for plecos I do not drop just one of two big chunks into a tank. Instead I make many smaller pieces and scatter them. I try to make food available all over a tank. When I have youngsters born in a tank growing, I usually feed different size foods. Often what fry need is much smaller than what the bigger fish want and often the bigger ones ignore the teeny morsels the fry want and which they can find more easily.
It is as hard for a big fish to eat powdered food as it is for the fry who wants to try eating the bigger bites that their larger species mates can and do eat.
