I just used "Industrial Strength Extreme" Velcro from Lowes on a project due to time constraints (would rather have made a metal bracket).
I was impressed enough that I'll use it again in the future.
Just make sure you clean the surfaces thoroughly with something like isopropyl alcohol and allow it 24hrs to cure before pulling on it.
IMO, the best sound comes from mounting the speakers at ear level on each side of the cab.
If newer shmups with quality soundtracks will be played on the cab, I'd put them on the outside of the cab on each side of the monitor (in their current housings).
If only playing classic arcade games, I'd put them below the monitor for cosmetic reasons. Maybe make a speaker grill to put in front of them covered with grill cloth.
Old games didn't have very good speakers in them anyway.

EDIT:
if I take the speaker out of the housing and drop them into the cabinet would the sound quality go way down?
Yes, unless you build a housing of the same volume of air space into the cab.
Any ports would also need to be replicated.
(I did this on the Neo Geo cab linked in my sig)