Thank you for reporting the issue. I could reproduce this issue as well. But, I needed to set the speed higher than 5000% to reproduce this bug, instead of 500%. Could you tell me your source audio format (such as frequency, sample rate, duration, and bitrate) that you used to reproduce this bug with 500% speed change? I reported this to our bug database, but I would like to have better understanding of this bug. Meanwhile, I found a workaround of this issue. Please add Volume effects from Effects panel to all clips you would like to change their speeds. You can keep Volume effect to 0dB so there is no actual change in audio. I could encode with this workaround. Hope this workaround works for you as well.
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