I haven been testing with some ryan addams music: http://www.archive.org/details/ra200...-22.flac16.sbd
And a whoosh sound effect file.
I also noticed a difference in sound but i did not relate i to a delay in sound. If you turn of either left or right source you will also hear a part of the other source somewhat softer on the other side. I blamed it on that but yours is also a reason.
Unfortunately you can only fill the buffers of one source at one time.

Possible fix: advance source one so it in sync with source 2.
Possible fix2: alSourcePlayv(2,@Sources);