This paper proposes an on-line audio watermarking system for broadcast monitoring. The designed watermark (WM) encoder is a nonlinear data adaptive system that performs perceptual embedding. It allows working at very low watermark-to-signal ratio (WSR) levels thus preserves the inaudibility. The developed decoder adopts wavelet de-noising for blind watermark extraction and it is capable of watermark decoding while establishing the synchronization between the transmitter and the receiver. Unlike the published watermarking schemes, it is shown that the introduced WM embedding scheme minimizes false alarm ratio by adaptively controlling the WSR. Furthermore it integrates synchronization and WM extraction into one processing step resulting in an on-line decoding scheme suitable to audio broadcast monitoring. The proposed system is robust to Digital-to-Analog (D/A), Analog-to-Digital (A/D) conversions, compression and noise as well as the attenuations that arise from FM broadcasting and acoustic transmission. Performance under Stirmak attacks is also reported. It is shown that error free transmission of 24 bps is achieved at around WSR=-32 dB when the PSNR is around 50 dB. Granularity of the system is 0.6 s thus it is capable of tracking short audio clips i.e. commercials.