We present the study of the Leggett collective oscillations with energy \(\omega _0\) of a superconductor plasma in two-gap superconductors MgB \(_2\) , MgB \(_2\) +MgO and Mg \(_{1-x}\) Al \(_{x}\) B \(_2\) with various doping level. We revisit our previous tunneling spectroscopy data to find the \(\omega _0^2\) dependence on \(\varDelta _{\sigma }(0)\varDelta _{\pi }(0)\) : (i) observed experimentally, and (ii) simulated using the reduced coupling constants \(\lambda _{ij}\) estimated from the experimental \(\varDelta _{\sigma ,\pi }(T)\) dependences. The comparison of both results at \(T \ll T_c\) shows the monotonic \(\omega _0^2(\varDelta _{\sigma }\varDelta _{\pi })\) behaviour, and the simulated \(\omega _0\) values tending to the Sharapov et al. predictions, those well exceed the experimental \(\omega _0\) values. We conclude on the substantial softening of the Leggett mode energy \(\omega _0 \rightarrow 2\varDelta _{\pi }(0)\) in Mg \(_{1-x}\) Al \(_x\) B \(_2\) compounds.
