I am unaware of any places in Danang. When I go there I am only looking a beach, sun, sand, and seafood.
For Hano, iIf we are speaking about only headphone related items there are a few places but fewer than just a few years ago since several SVHouse locations have closed.
My favorite place is a small shop called Ido. Very nice folks and interesting selection of IEM and headphones plus associated items.
https://ido.com.vn/
There are a couple of SVHouse locations stil open, check here:
http://loa.com.vn/
For speakers there are several high end shops around town. I would start of on Hai Ba Trung street near the intersection of Pho Hang Bai sort of behind Tran Tien Plaza. There is also a place on Pho Hang Bai called Digital World Hanoi that some times has decent headphones.