Clinkle has been the talk of Silicon Valley for the past year, although few of the consumers who will make up its hoped-for user base have even heard of it. That is in part by design. A payment app co-created by Duplan, it has been in “stealth” mode for more than three years, its unique selling proposition a closely-held secret. … A few details have leaked, however. Those who have used Clinkle say it’s an exceptionally well-designed, simple-to-use mobile wallet app that utilizes a high-frequency sound to transmit money between users in close proximity — no phone-bumping, QR code-scanning, texting or dongle-swiping required. … Last June, Clinkle announced a $25 million round of seed financing from a dozen top venture capital firms and angel investors — the largest early investment raised in Silicon Valley history. A few months later, the company raised $5 million more from Stanford’s StartX fund as well as Virgin CEO Richard Branson. Duplan, a first-time founder who was barely of legal drinking age, seemed full of promise. … But nine months later — during which time several rounds of layoffs have been announced and rumors swirled of turmoil inside the company — the app still has yet to launch publicly.