The Creative Commons organization has a page on Legal Music For Videos which states (emphasis mine):
Under CC licenses, synching the music to images amounts to transforming the music, so you can’t legally use a song under a CC No Derivative Works license in your video.
This corresponds to the "synchronization" provision in section 1(a) of CC-BY-SA 3.0 (and is also included in every CC license, I think):
For the avoidance of doubt, where the Work is a musical composition or sound recording, the synchronization of the Work in timed-relation with a moving image ("synching") will be considered a Derivative Work for the purpose of this License.
While it may not at first be obvious what level of coordination between music and video is necessary to satisfy the definition of "synchronization", I believe relevant case law demonstrates that any combination of music and video presented in a simultaneous format activates this clause. The term "synchronization" is used commonly in a closely-related copyright context: a synchronization license, of which the American Society of Composers, Authors and Publishers says
A synchronization or "synch" right involves the use of a recording of musical work in audio-visual form: for example as part of a motion picture, television program, commercial announcement, music video or other videotape. Often, the music is "synchronized" or recorded in timed relation with the visual images.
It is unlikely an accident that this is so close to the synchronization language used in the Creative Commons text. I expect the intention of the Creative Commons license is, "Any use that would normally require a synchronization license from the author of a musical work is a use that generates a Derivative Work." Everything I've read about synchronization licenses indicates that they are necessary for any simultaneous combination of video and music. It is "synchronization" in the most basic sense; think "merely simultaneous" rather than "creatively coordinated".
Notably, in Abkco Music v. Stella Records, the court found that synchronization rights were required even to synchronize music with arbitrary images in a karaoke machine's visual output:
...CD+G's display only the lyrics, whereas karaoke discs display some video image, such as a sun-drenched beach, behind the song lyrics. Under the Copyright Act of 1976, 17 U.S.C. § 101 et seq., the producers and distributors of karaoke versions of songs must acquire synchronization or "synch" licenses from the copyright owners of the songs...
When the Creative Commons synchronization clause applies (which it appears to do even in the most trivial of video synchronization), the entire video becomes a derivative work, and the share-alike license terms apply to the new derivative video, per section 4(b) of CC-BY-SA 3.0.
In short, using CC-BY-SA music in a video requires you to license the entire video under CC-BY-SA.