Instabilities of the human voice source appear in normal voices under certain conditions (newborn cries, vocal fry, creaky voice) and are symptomatic of voice pathologies. Vocal instabilities are intimately related to bifurcations of the underlying nonlinear dynamical system. We analyse in this paper bifurcations in 2-mass models of the vocal folds and study, in particular, how the incorporation of the vocal tract affects bifurcation diagrams. A comparison of a simplified model (Steinecke & Herzel, 1995) with an extended version including vocal tract resonances reveals that essential features of the bifurcation diagrams (as e.g. frequency locking of both folds and toroidal oscillations) are found in both model versions. However, vocal instabilities appear in the extended model at lower subglottal pressures and even for weak asymmetries.