Press and hold the Command-Option-P-R keys immediately after you turn on your Mac.
Hold these keys down for at least 20 seconds to ensure that your Mac completes the process correctly. (Source)
Long term Mac fans know that to reset the NVRAM you would wait to hear two startup chimes. Gone is that instruction.
The Macintosh startup chime is a single note or chord simply known as “the startup sound”. The sound used differs depending on the ROM, which greatly varies depending on model type. This short sound is played when an Apple Macintosh computer is turned on. The sound indicates that diagnostic tests run immediately at startup have found no hardware or fundamental software problems. (Via)
In the post 2007 (after iPhone) Apple, all new products (iPhone, iPad, AppleTV, Apple Watch) feature no startup sounds – so the writing was on the wall. For long term Mac enthusiasts this is an end of an era as Macs have been chiming since 1984 – and was predicted to last until 2815.
Over the years the chime has changed. The first being a “bong.” Quadras brought in a chord composed in a C major on a Korg keyboard. It was pitch shifted a few times. Power Macs brought in an electronic jazz guitar strum. The 20th Anniversary Macintosh had a poppy remix of the classic bong as a tribute to the original Mac. But all Mac computers post the iMac G3 have had the same sound – an F-Sharp cord sound modulated from a PowerMac startup sound.
(Updated to reflect a Wall-E reference)