It is needed to configure last line aligned to left for Score in Musixtex (it is needed to move the final bar to left from red line to orange line approximately). The basic code is as follow:
\documentclass[aspectratio=169]{beamer}
\usepackage[T1]{fontenc}
\usepackage[spanish]{babel}
\usepackage{musixtex}
\begin{document}
\begin{frame}[t]
\frametitle{?`Qué es \underline{Musixtex}?}
\centering
\begin{music}
%\let\addspace\relax
\parindent0mm
\nobarnumbers
\instrumentnumber{1} % a single instrument
\setstaffs1{1} % with one staffs
\generalmeter{\meterC}
%\startextract
\startpiece
\NOTEs\wh{c}\en\bar%
\NOTes\hl{g}\hu{e}\en\bar%
\NOtes\qa{fghi}\en%\bar
\alaligne
\Notes\cl{qpon}\cup m\ccu l\clp k\ccl j\en
\setdoubleBAR
\endpiece%\endextract
\end{music}
\end{frame}
\end{document}
The current output is as follow:
Just for clarification. After feedback the solved code is as follow:
\documentclass[aspectratio=169]{beamer}
\usepackage[T1]{fontenc}
\usepackage[spanish]{babel}
\usepackage{musixtex}
\begin{document}
\begin{frame}[t]
\frametitle{?`Qué es \underline{Musixtex}?}
\centering
\begin{music}
%\let\addspace\relax
\parindent0mm
\nobarnumbers
\instrumentnumber{1} % a single instrument
\setstaffs1{1} % with one staffs
\generalmeter{\meterC}
%\startextract
\startpiece
\NOTEs\wh{c}\en\bar%
\NOTes\hl{g}\hu{e}\en\bar%
\NOtes\qa{fghi}\en%\bar
\endpiece
\hsize70mm\hoffset0mm\parindent0mm
\startpiece
\Notes\cl{qpon}\cup m\ccu l\clp k\ccl j\en
\setdoubleBAR
\endpiece%\endextract
\end{music}
\end{frame}
\end{document}
\centering
here.