The email from youtube is just an automated email because the soundtrack was content matched. Depending on the record label youtube handles these things in a couple ways, the coolest and most common way is like what happened with your video, they simply identify the song below the description box and leave it at that. And sometimes the video may be blocked in certain countries, for example I have a video with a song that was content matched and it is blocked in Germany, I’m guess this all has to do with copyright laws in Germany.

The other way and something that I’ve never actually experienced myself is they may remove the audio completely. When this happens it might count as a strike but I’m not really sure.

Overall its nothing to worry about its not a ‘strike’ on your account. I think if you were a partner channel then it might be a bigger deal since it is someone else’s content and you are getting paid for views.