I've also been searching for that double-bong for years.

Last time I asked, user hackmiester pointed me to https://goughlui.com/2016/05/03/project-the-definitive-colle...

The "Texas Instruments DSP based Modems" linking to USR-Sportster-bong-bong.wav is pretty close to what I remember.

edit: hackernudes reply is perfect. The youtube auto generated subtitles are pleasing too.