Solution to move async to the whole function in the callback:

mediaRecorderAudio.addEventListener("dataavailable", async (stream) => {
    // Send stream data while mediaRecorderAudio is active
       let blob = new Blob(stream.data, { type: "audio/ogg; codecs=opus" });
       let buffer = await blob.arrayBuffer();
       let data_to_send =  new Uint8Array(buffer);
       socket.emit('socket_audio',JSON.stringify(data_to_send))
});
mediaRecorderAudio.start(100);