The Theater Alte Brücke in Sachsenhausen offers live and improv theatre and readings on it's Facebook page. The theatre's website shows an overview of the latest content. (Offers in German, some plays in English)
You can listen to recordings of the hr-Sinfonieorchester on Youtube. Additionally, two musicians will play short live concerts from the hr-Sendesaal Frankfurt every day from Monday to Friday. (Offer in German, but no German language skills required)
The series "30 Minutes of Organ Music" by Martin Lücker, organist at St. Katharinen at the Frankfurt Hauptwache, has been made available online through videos on Youtube. (Offer in German, but no German language skills required)
In a "Lockdown Edition", the Jazzkeller offers talks and music on its Youtube Channel (available in German and English).
In the next couple of weeks, the Mousonturm's "Secret Salon" will take place online: Every Thursday from 9 pm onwards various DJs and musicians will livestream on Youtube. (No specific language skills required)