MD11 Captain reports inadvertently turning of IRU #1 while attempting to adjust the speaker volume on the overhead panel. Flight continues to destination.

Narrative

In cruise at FL370; I was pilot flying and had turned the volume control up on my speaker not realizing I had turned it almost all the way. ATC made a transmission to another aircraft and as it was very loud; I quickly reached up to turn the volume down and realized as I did so that I had inadvertently selected the #1 IRU off. I passed control of the aircraft to my First Officer; got out the QRH (Quick Reference Handbook) and complied with the Abnormal Alert 'IRU _ Fail' checklist. At no time did the aircraft deviate in any axis; nor was there any course deviation. The rest of the flight was normal. This has happened before to other crews on this aircraft. I was aware of the problem of the speaker volume control knob being adjacent to the IRU control panel. The knobs are shaped differently; something I was well aware of. This was a self-induced problem. Maintenance was notified at the arrival gate and reset the IRU to the gate position for the next outbound flight. It may help to have the IRU knobs designed to be pulled before selecting OFF ... that or a guard of some sort over the IRU knobs. Bottom line ... lesson learned ... no matter how familiar with the aircraft; look before one moves a switch or knob!