How to merge both audio channels into left channel?

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http://askubuntu.com – My headset broke recently and now only left side of it works. In some songs, there are parts that are only played through the right side and since my right side doesn't work, I can't hear them. I want to set the sound so it only plays on the left side. I tried changing the balance settings in sound preferences, but it only mutes the right channel, doesn't make it play through the left channel. How can I do this? (HowTos)