The PC speaker is normally meant to reproduce a square wave via only 2 levels of output (the speaker is driven by only two voltage levels, typically 0 V and 5 V).
For example, if your payments for March and July 2010 were each one month late, the report may continue to show (for seven years from the date after each payment was due) that you were 30 days late twice in 2010, even though the tradeline for that account also shows your payments for the rest of 2010 were made on time.
Late payments remain on your credit report for seven years. The further you put your missteps in the rearview mirror, the less they hurt your score. If your report is otherwise pristine, a single late payment should stop affecting your score in six to 12 months.