Yeah, it all depends on which definition of hazing you use.
However, more importantly, why are you doing anything if it doesn't serve a clear purpose and have solid results?
There are places where road trips and scavenger hunts are considered hazing (thankfully, not my campus). There are times where singing can be hazing, but there are also times where making them work out until they're sore all over isn't.
Other posters have covered why singing, in your situation, is hazing. I'd consider it hazing too.
As for how you can make pledges work out until they're sore while not having it be considered hazing: they show up and work out with some of the more athletic brothers.