Links stolen from the Martin Luther King J. Papers Project at Stanford University (whose home page annoyingly plays audi with no recourse).

"Letter from Birmingham Jail" - 16 April 1963
This version of appeared in his 1964 book Why We Can't Wait. View the statement that prompted this letter.
Acrobat PDF | Quicktime | Realmedia

March on Washington for Jobs and Freedom - 28 August 1963
The "I Have a Dream" speech. Available in multiple languages.
Acrobat PDF | Quicktime | Realmedia

Acceptance Speech at Nobel Peace Prize Ceremony - 10 December 1964
Address delivered in Noble Peace Prize Ceremony in Oslo.
Acrobat PDF | Quicktime | Realmedia

Beyond Vietnam - 4 April 1967
Address delivered to the clergy and laymen concerned about Vietnam, at the Riverside Church.
Acrobat PDF | Quicktime | Realmedia

I've Been To The Mountaintop - 3 April 1968
King's last speech.
Acrobat PDF | Quicktime | Realmedia