My computer never came with speakers so I’ve always used headphones when on my computer and 放つ sounds perfectly fine to me. Maybe it could be something wrong with your headphones.
Just listened to this on both $15 earphones and $150 headphones, and I see what you mean. I can hear はなつ at first, but after a while it becomes harder to hear, and easier to hear かなつ. It’s probably just that the “h” sound is quite breathy, so your mind starts hearing it as a “k”.
It may also be a hardware thing. Sometimes there’s a pop when playing audio, which seems to be present here. That may just be me though.
Good point. Could be a bit of a pop in the recording process. I won’t worry about if if sounds fine to others. I know these things can be a bit subjective.