Dropout kara no Saishuushoku Saki wa, Isekai no Saikyou Kishidan deshita: Wakeari Violinist, Maryoku Kaifukuyaku ni Naru

Masumi has fallen off the road to stardom and now she earns a meager living playing the violin, her specialty. On her way home from a part-time gig, she falls down a flight of stairs and finds herself transported to another world! She is immediately captured by a squad of knights and accused of being a spy! Naturally, Masumi tries to explain herself to the Knight Commander, Arc, but his suspicions remain unanswered… until Arc notices her violin, and asks her to work as a “musician” for his Order of knights until she is cleared of the espionage charge.

To her surprise, Masumi finds herself in a world where playing music helps to restore magical power!

Masumi, with nowhere else to go, accepts the offer and immediately sets to work. However, she learns that the greater the knight's magical power, the more demanding it is on the musician's skill. And then she discovers that Arc leads a group of knights who have been shunning other musicians because they are the “most powerful” ones.

(Source: Over-Lap, translated)