Acoustic event detection (AED) aims to identify both timestamps and types of multiple events and has been found to be very challenging. The cues for these events often times exist in both audio and vision, but not necessarily in a synchronized fashion. We study improving the detection and classification of the events using cues from both modalities. We propose optical flow based spatial pyramid histograms as a generalizable visual representation that does not require training on labeled video data. Hidden Markov models (HMMs) are used for audio-only modeling, and multi-stream HMMs or coupled HMMs (CHMM) are used for audio-visual joint modeling. To allow the flexibility of audio-visual state asynchrony, we explore effective CHMM training via HMM state-space mapping, parameter tying and different initialization schemes. The proposed methods successfully improve acoustic event classification and detection on a multimedia meeting room dataset containing eleven types of general non-speech events without using extra data resource other than the video stream accompanying the audio observations. Our systems perform favorably compared to previously reported systems leveraging ad-hoc visual cue detectors and localization information obtained from multiple microphones.