Robert Picardo and Jeri Ryan did at least some of their own singing, including the duet "You Are My Sunshine."
The sperm and ovum showed during the Doctor's lecture is a screen shot taken from the film Look Who's Talking (1989).
The scene in the ready-room where Seven helps Janeway put on her pip was removed from most airings on US stations during its first run (although the scene is present in all syndicated reruns and on the DVDs). No explanation for this was ever given. However, the scene was intimate and flirtatious so the most likely explanation is because it was between two women, and Star Trek has often shied away from suggesting such relationships.
During her date with Lieutenant Chapman, Seven of Nine refuses having a glass of champagne, since synthehol affects her cortical functions, which had occurred earlier in the season when she had a glass of champagne in engineering in Timeless (1998).
The storyline involving The Doctor, Seven and Tom is based on the 1913 play "Pygmalion" by George Bernard Shaw and its 1956 musical adaptation "My Fair Lady," with the Doctor as Henry Higgins and Seven as Eliza Doolittle.