Mixing:

General principles

Mix 8->16 samples at once. If you have to mix an odd number, jump into the
table at an offset

Instructions:

 Add <FRACTIONAL>, <DELTAFRACTIONAL>
 Mov AL, [ESI]
 AdC ESI, <DELTAOFFSET>
 Mov EDX, [EDI]
 Mov EBP, [EDI+4]
 Mov EBX, [EAX]
 Mov ECX, [EAX+<StereoOffset>]
 Add EDX, EBX
 Add EBP, ECX
 Mov [EDI], EDX
 Mov [EDI+4], EBP