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.
If you miss a payment or make a late payment within 30 days of the original payment due date, that goes on your credit report and stays there for seven years. Once you become 90 days late on paying a bill, your account is closed and sent to a debt collector, which also stays on your credit report for seven years.