As the only person in my friend group who does not still live with their parents, we often end up having drinks at my apartment. For the most part, it's a good time and my friends are great guests. However, there is one guest (who is a friend of a friend) who has a habit of coming in and promptly playing her own music on her phone (very loudly) as the party soundtrack every time she comes over. This is frustrating as I often have my own music playing and she will play her music right over mine so we have both going at the same time. In addition, if my music is just loud enough that she can't play hers over it, she will change it music to something she likes without checking first. I use my Macbook for this purpose, and I am hesitant to let people just pick it up and use it, especially when they're holding a beer as they look through my music library! We also have very different tastes in music — which adds to my annoyance as she is critical of my taste and is vocal about it. How can I get her to stop doing this? Other guests have noticed this and feel that I should say something, but I don't want her to feel unwelcome or cast a damper on the party.