Hi Tony,
there are issues with Music service authentication which hopefully we can solve given time.
For your use case you are already authenticated and playing Spotify. Assuming you started it from the Sonos controller then you can resume it.
There is a SoCo module called snapshot which saves the current state, and then after you have done something (eg play your custom mp3 file, you can restore the previous state.
However, if you started the music using Alexa then Sonos uses a cloud queue, and up to now this can't be re-started.
I think the same applies if you start the music from the Spotify controller.
There may also be a time out on authentication if you are thinking of a long pause before you resume.
I use the basic approach with my doorbell so it pauses what is playing, plays a doorbell mp3, then resumes.
This works using snapshot - with the exceptions mentioned above.
Hopefully this helps
Cheers David