In this paper we propose a novel method for audio watermarking which is appropriate for real-time applications. Here the energy proportion of two successive low-middle frequency bands of each audio signal frame is computed and through the use of index modulation technique we embed the watermark bits into these proportion values. Low complexity is the most important characteristic of the proposed method making it appropriate for real-time applications. The method is a blind one, that is it does not need the original signal for extracting watermark bits. Experimental results have shown that it has a very good robustness against common watermarking attacks such as MP3 compression, low pass filtering, resampling, requantizing and adding noise.