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.
Original lenders and creditors show payment default on your credit reports for 7.5 years. When you first miss payments to a creditor or lender, you will see lat pays show up in 30 day blocks of time on your credit report. If the accounts remain unpaid long enough, those 30 day incremental late pays turn into a charge off reporting on your credit. Charge offs normally occur within 180 days of consecutively missed payments (they can happen sooner than 180 days).