。
She grumbled that my salary was low.
Literal
She [topic-は] my salary [subject-が] cheap [quotative-と] muttered.
ぶつぶつ言う is the standard expression for muttering, grumbling, or complaining under one's breath — built on the mimetic ぶつぶつ, which evokes the sound of half-articulated complaints. Note the structural neatness: the embedded clause 給料が安い ('salary is low/cheap') uses が because within that clause, 給料 is the subject; outside, 彼女 is the topic of the whole sentence. The と quotes the embedded thought, and ぶつぶつ言う reports it. 安い for a salary literally means 'cheap' — Japanese readily applies the cheap/low axis to wages, prices, and (more rarely) people in derogatory uses.