Get a small Klipsch center channel speaker. It's horn loaded; and will
overcome the sound of the treadmill. Connect this to the left speaker
terminal on the 15 y/o receiver. By old fashioned clamp-type, do you
mean little push-tabs that you pinch bare wire into? Now, connect an
audio cable to the left 'tape' or CD input. Buy a 'Y' adapter for the
DVD player end of that cable. Now you can combine the left, and the
right channel sound coming from the DVD player into one speaker.