for example if you want to play new music when the brute appears, grab the servant_brute.ent from the amnesia/redist/entities/enemy/servant_brute folder, make a copy of it, rename it like servant_brute_newmusic.ent, open it up in notepad. search for these lines in it:
<Var Name="DangerMusic" Value="dan_brute.ogg" />
<Var Name="DangerMusicPrio" Value="0" />
<Var Name="SearchMusic" Value="search_brute.ogg" />
<Var Name="SearchMusicPrio" Value="0" />
<Var Name="AttackMusic" Value="att_brute.ogg" />
<Var Name="AttackMusicPrio" Value="0" />
These are user-defined variables. If you want to change the music, put your music file (in .ogg format) to the amnesia/redist/music folder, and simple change the
<Var Name="DangerMusic" Value="dan_brute.ogg" />
line for example to
<Var Name="DangerMusic" Value="yourmusicname.ogg" />
(yourmusicname is the name that your music has in the music folder) and then do the same with the other two musics, if you have new ones for those also.
There is one when the enemy activates, when it searches for you and when it sees you.
That's it. I hope you understand
There are other ways to change the monsters' music, but this is the easiest way.
For your other question. It's not very different. Copy the oil entity, rename it potion_oil_small for example, open it.
Search for this line:
<Var Name="OilAmount" Value="25" />
and change the "25" to "10" for example.
Almost the same for the other potions, i think you can do it yourself
REMEMBER, always make copies of these entities, do not overwrite any original files, if you want to release any custom stuff!