We don't have synonyms for line level yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are line-level, find level and blind level.
(N) :
the specified strength of an audio signal used to transmit analog sound between audio components such as CD and DVD players, television sets, audio amplifiers, and mixing consoles.
(N) :
the specified strength of an audio signal used to transmit analog sound between audio components such as CD and DVD players, television sets, audio amplifiers, and mixing consoles.