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.
Your late payments will appear on your credit report for 7 years. Their impact is significant in the first 12 months and decreases from then on, but they will continue to impact your score for quite some time.     As a general rule of thumb, a 30-day late will drop your credit score by about 50 points.