This month marks Stephen Sondheim’s 90th birthday, and there is much to celebrate regarding his music, his lyrics, and his influence over the shaping of musical theatre for the last sixty-plus years. As Mr. Sondheim becomes a nonagenarian, Broadway Records provides us with a unique way to explore his music. The new album “Losing My Mind: A Sondheim Disco Fever Dream” offers an “off-the-beaten-path” assemblage of many of his well-known (and lesser-known) tunes dished up in a decidedly clever format that evokes mirror balls, Donna Summer, and Studio 54.