I have the same issue since updating to ios 11.4 last night. Along with the beep, my bluetooth speaker stops playing music and I have to manually hit the volume up button to play more music.
Since this is a new issue with this update and my music played fine yesterday with the previous ios I really don't want to hear from Apple about how this is an issue other than with ios 11.4. It would be completely unreasonable to suggest that the problem is with the speaker as it worked find with ios 11.3 yesterday. This has effectively bricked my bluetooth speaker and I would find it unreasonable to suggest that I purchase a different speaker to resolve this issue.
A timely fix from Apple would be appreciated. I find music vital to helping me get through my workday smoothly and streaming music throughout my office over my speaker is the way I do that. As I said, my speaker is now an expensive brick that will only play one song at a time with an annoying beep at the end. Unless, that is, I am willing to walk to the speaker and hit the volume button after every song.