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.
A late payment can stay on your credit report for up to seven years. This is the standard time limit for most items. Only a bankruptcy can stay longer -- up to 10 years in some cases. But for late payments, seven is the magic number. You can think of your credit reports as a record of your previous debt payments.