The more you listen, the more interesting it gets.

Literal

Listen-[if-ば] listen-as-much-as increasingly interesting-becomes.

聞けば聞くほど is the classic ~ば~ほど pattern meaning 'the more you do X, the more Y.' ますます reinforces the escalation. 面白くなる uses the く-form of an い-adjective plus なる to express 'becomes interesting' — a core adjective-change pattern.