Binaural is all about recreating the head‑related transfer function, so the first thing is to get each source in its own stereo pair, not just panned flat. Record the leaf rustle with a close pair of microphones so the subtle Doppler shift from wind is captured. The siren should be a distant source with a low‑pass filter to emulate attenuation, but keep a faint reverberant tail so it feels like it's outside the alley. Then layer the city heartbeat—traffic rumble, footsteps—as a low‑frequency ambient field, maybe recorded on a room mic to get the diffuse energy. When you mix, keep the dynamic range tight: let the quiet elements sit just above the hiss, the midrange siren bite in the center, and the bass rumble as the background. Use a narrow EQ on the siren to avoid masking the rustle, and maybe a touch of stereo widening on the ambient field to give depth. Finally, apply a very subtle HRTF convolution per source so each cue feels anchored to its direction. That way the listener perceives a living space, not just a flat collage.