After searching out my old book box and digging through it, I got my fingers on the book I remembered: Das Aleph System is the German translation of Halo by Tom Maddox.
The scene that lingered in my mind was from chapter 9 Virtual Café:
The percussionist hit her keys, a slow shuffle beat played, and a fill machine laid a phrase across the beat: "Bam! Ratta tatta bam! Bam bam! Ratta bam!" The stick player joined the drummer with his own lo-beat fills—walking bass, sparse piano chords, slow and syncopated. The horn player stood with his eyes closed, apparently thinking. After several choruses, he started to play.
He began with hard-edged saxophone lines, switched to trumpet then back to saxophone, played both in unison, looped both and blew electric guitar in front of the horn patterns. Scatting voices laced through the patterns—Gonzales couldn't tell who was making them. The drummer's hands worked her keyboards, her feet the various kickpads below her; the song's tempo had speeded up, and its rhythms had gone polyphonic, African.
About the Book
The following synopsis is taken from project.cyberpunk.ru
The latter half of the twenty-first century: a time when artificial intelligence is going to transform humanity.
Free-lance data-auditor Gonzales is given a contract by the Sentrax Corporation to monitor goings-on aboard Halo, an orbiting colony controlled by an artificial intelligence named Aleph. Seems that badly injured Jerry Chapman will dies unless Aleph can preserve his mentality inside its own computer reality. Assisting Aleph and Gonzales are computer reality interface expert Diana Heywood and Halo's Interface Collective, an association of talented weirdos. Gonzales's Sentrax boss, Traynor, opposes the project and pulls the plug just as Aleph, Gonzales, Diana, Jerry, and the Collective are hooked together. Aleph, annoyed, retaliates by switching off the functions that sustain human life aboard Halo; thus Traynor and Sentrax are forced to back down.
A critique about it can be found here, describing it as
Maddox's first novel offers a provocative mixture of virtual reality, artificial intelligence and mystical philosophy. While raising interesting questions about the nature of perception, cognition and self-awareness, however, he fails to explore them fully.
The story's tension is uneven, and motivations are not well defined. Scenes occurring within the computer are distinguished by vivid imagery and poetic turns of phrase, but Madddox falls back on vague mysticism rather than offering specific ideas about the nature of intelligence.
It is freely available online at Manybooks and on the author's website (Internet archive link).