I've decided to try and work out what the unknown akao operations are by just forcing the function to be called in assembly. The latest confirmed is operation 0x10. It is a "Play music" operation. You don't need music to be set for a field in order to play music. Operation 0x10 takes the first parameter as the music ID (which is -1 compared to the field music IDs... because there isn't two "none". In other words, "Farm Boy" is ID 0x20 (32) and not 0x21 (33). )
If you search function call 00740D80 you'll see that this is akao (and you'll see that there are a lot of calls made to akao from hard code - including the Game Over music volume set). It is fed in 9 parameters. If the desired music is already playing, no change will be made.
The music play is especially important to begin the credit music at game end (as well as other musics that have to be set, like the opening credits). You can see this at 7A79B2.
The world map music is set at 75e61a. You can see it pulls the ID from a table (the Great Crater music, the 2nd world map music - post Meteor - is at 96D5A0).
0x10, 0x14: Play music assigned at parameter 1. Both of these are exactly the same operation.
0x18, 0x19: Play music assigned at parameter 1, but resume if previously played (if the music has been played before, carry on from last point). This will work even if the module changes from field to world map (and is probably remembered regardless of module). 0x18 and 0x19 are exactly the same operation, so just make Makou have one operation, rather than two.
The world map music uses 0x18, because it has to resume.
0x00 to 0x0F don't exist. The funtion actually deducts 16 (0x10) from the first parameter immediately
So basically, 0x10 is the first ID that can be used.
Also, the main operation to be carried out occurs at 740DB9. From that you can also see which operations are just calling the same part.
Confirmed as doing nothing (function jumps to end). These can safely be removed from Makou Reactor:
0x11, 0x12, 0x13, 0x16, 0x17, 1A - 1F, 2C - 2F, 31 - 97, 9E - 9F, BE - BF, C3 - C7, CB-D3 - EF, F2 - FF.
Additionally, these are all the duplicates:
0x10 is the same as 0x14 [Note - this is also what "Play Music" Opcode F0 does. It just calls akao with this operation].
18 is the same as 19
20 is the same as 24
21 is the same as 25
22 is the same as 26
23 is the same as 27
I have checked these against the wiki.
On the wiki it doesn't show 0x30 as used - but it is.
0x30 plays a sound effect - but always centred panning. It takes parameter 1 as the effect ID (whereas 0x28 - 0x2B use the first parameter as the left / right balance, and the 2nd parameter as the effect). It does not appear to be using Channel 1 - 4, which means this could be considered channel 5? It should be noted that there appears to be no way to stop this effect once it has started. You can change to another effect, but effect #0 will not silence it. My guess is that 0x30 is meant to be used when you have an ambient sound effect in the background that isn't ever meant to be terminated. Leaving the field module will terminate it, same as any other effect.
0x20: Play a sound effect (will be terminated if another effect is played on channel 1-4) [param1=Panning, param2=Effect ID]
This *seems* to be the same as 0x28 with one major difference: If any other sound effect is played from Channel 1-4, the sound effect playing from 0x20 will be terminated. So this one, as far as I can see, is simply a "play effect - but terminate if ANY other effect is played". However, 0x30 will not affect this.
0x21 - 0x23 do the exact same thing as 0x20, except they take in additional parameters. What these parameters do, I don't know. I can't get anything to sound different by using them. They may be broken on PC - or simply serve no function.
0xC8, 0xC9, 0xCA have been completely removed from PC version, but are still needed in Makou as per PSX game.
http://wiki.qhimm.com/view/FF7/Field/Script/Opcodes/F2_AKAOI have updated the Wiki.
Finally: 0x15. Not sure what this does. But at 740E49 it is jumping to Aali's driver. So he'd have more idea. It does not take any parameters.