Different signal propagation velocities can be advantageous for passive tracking. For example, electronic and acoustic sensors can be used in conjunction to localize objects emitting electromagnetic waves and sound. In a heterogeneous passive sensor setup involving electromagnetic detection and acoustic bearing sensors, observability is studied, even for the case that the signals are not emitted simultaneously. It is shown that observability can be established and target maneuvers are not necessary. Finally, a numerical analysis of the Cramér-Rao lower bound is performed to verify the results.