Additions:
======rFactor PLR File======
?? Not sure but I think if this is set to 0 then you just get general background sounds in the monitor screen and you will not hear any of the other cars passing the pits, etc.
Not tried altering any of these but changing the Airhorn values will probably reduce their effect enough to stop them being as annoying as they are at some tracks (eg when in the pits).
Deletions:
Query - not tried but perhaps setting this to 0 will disable those annyoing air horns so loud at some tracks!!
"Pit Volume" is perhaps another way of dealing with those annoying air horns!
Not tried altering any of these.