I wasn’t able to find a whole lot of material relating to Smiles of a Summer Night, but I did come across a few videos that might be useful.
The Bergman film is on YouTube in full:
You can turn on captions using the “CC” button at the bottom.
I couldn’t find a free stream of the A Little Night Music film, but there is a New York City Opera performance from 1990 on YouTube:
The classic rock band Heart released a song called “Smiles of a Summer Night” as a Best Buy-exclusive bonus track on their 2012 album Fanatic. Here is a blog post with a stream:
There is also a Desperate Housewives episode called “Smiles of a Summer Night”. I’m not really sure how much it has to do with the film. You can stream it on NetFlix. Here is a very detailed article on it from the Desperate Housewives wiki:
Here is an NYT article discussing Smiles in relation to A Little Night Music:
“Send in the Clowns” has, of course, been covered by Judy Collins, Barba Streisand, and many others. Here is Krusty the Klown singing it on The Simpsons:
For the record, here is a performance of Mozart’s Eine kleine Nachtmusik: