I am building a stereo speaker cabinet and have some questions about birch plywood.

1.) Is marine plywood much heavier than non marine plywood for a give size (3/4", 1/2",etc)?

2.) Is marine plywood much stronger than non marine? So could 1/2" marine have the same strength as 3/4" non marine?

3.) Most of the people I have spoken to use birch for the speaker cabinets. Many of them say to use 13 ply baltic birch. All I can find localy is 7 ply birch.
A.) Do I really need 13 ply or is 7 strong enough?
b.) If I use the marine will that not add strength so I can use 7 ply?
c.) What is the difference between baltic and other birch?

4.) Can you think of a reason other than looks people use the Birch?

I am hoping someone here can help with some of the answers as most people at the stores have no clue about the woods that are there.

Thanks

finished plywood