400 hours. Is that the standard time it takes speakers to break in? I think I heard that # for the Miflex caps, but that seems a lot for speakers.
In my experience, woofers need a good 50 hours of physical break-in, playing bass heavy "brown noise" or music under a bunch of heavy blankets gives them a solid workout to help fill out the bottom end.
The X-statiks are notorious for needing this kind of work out, as they sound really harsh when they are fresh.
Budget-level caps usually dont experience much burn in, and what there is usually happens within the first 100 hrs.
Good quality caps like Sonicap need about 200 hours, with gradual improvements over time, namely to soundstage, imaging, and softening of the edge they have when fresh.
High end caps, (especially oil damped/impregnated caps like Miflex, Jupiter, Mundorf, V-Cap, Duelund, etc) often need 400+ and some of them can be gradual improvement, or dramatic, like Jupiter and the V-Cap ODAM which can sound great one day but forward the next.
(As bypasses, the changes over time are usually more subtle, and less dramatic than when used as main capacitors.)
Most retail speakers won't benefit much from burn-in or break-in.
I guess that's a good news/bad news thing. Good news is that my little Encores are going to keep getting better. Bad news, I will drive my wife crazy leaving them on 24/7. The blanket idea is a good one though. 
Is there an ideal volume that is required?
For the first 50 hours of break-in, as loud as is tolerable.
Beyond that, any volume from a whisper up to normal listening levels will benefit the process. Some like to fluctuate the volume throughout the day, but a static volume is fine for burn in.