The problem of internal soliton oscillations or soliton-magnon bound states occurring in the quasione-dimensional antiferromagnet (CH3)4NMnCL3 (TMMC) is studied by approximated analytical methods. Above T&, besides the well-known Goldstone mode, we found a second bound state which is due to the coupling between in-plane and out-of-plane components of the spins when going beyond the sine-Gordon limit. For typical experimental conditions the frequency of this second bound state is very close to the bottom of the magnon band. Below T& using an interchain mean-field approach we also found two bound states: the Goldstone mode and a second state which originates from the pairing of m kinks described by a double-sine-Gordon equation. Out-of-plane effects for this mode are also included by means of perturbation methods. In contrast to the situation above Tz the frequency of this mode can be located in the whole gap between zero and the continuum. Both above and below T& the polarization of these second bound-state modes has the same orientation, which differs significantly from that of usual magnon modes.