Как отключить индикатор уровня громкости на экране -при изменении громкости в Linux Mint?

Да, вы правы. Проверьте определение memlock здесь

memlock 
maximum locked-in-memory address space (KB) 
This is memory that will not be paged out. It is frequently used by 
database management applications such as Oracle or Sybase to lock 
shared memory for a shared pool so that it is always in memory for 
access by multiple sessions.