One possibility is the tracking/audio head is dirty. Remove top cover of VCR, this head sits just to the right of the rotating video head. Use a cotton swab moistened with rubbing alcohol, or other video head cleaner and clean this head, especially the lower edge, as the tracking info is recorded along the lower edge of the tape (if memory serves). Another possibility is a problem with tape travel past this head. If the tape is lifting or moving up or down, even slightly as it passes this head, tracking will be lost, hence your blue screen & bad audio. This could be observed by playing a tape with the VCR cover off.
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