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You have a song where you'd like to remove the vocals or the lead guitar. It doesn't work on every song. It depends on how it's recorded or mixed. It doesn't work mostly on old songs or amateur recordings. But you can try.
First download Audacity, it's a free audio engin. You also have to download ASIO4ALL. You need ASIO (Audio Stream Input Output) for all recording programs for your PC. ASIO is a professional audio standard that provides for a low-latency audio stream from the computer. ASIO bypasses the KMixer entirely. ASIO is used by the recording industry as a standard.
Follow the instructions and install your ASIO4ALL.
Open your song. Split the stereo line and reset both lines to mono! Mark the part where you'd like to remove the lead. Look under Effect: set -> intervene.
Now you only have to highlight the basses.
That's all. No magic. It should work on most newer songs. You can remove vocals or lead instruments.
Good luck, and have fun. And remember! Using songs of other artists without their permission is illegal! So stay clean!