It is needed to adjust the octave symbol of Ioctfinup command. I try it using: \def\octnumberdown{\ppffsixteen8$_{ba}$}, but it doesn't works. The basic code is as follow:
\documentclass[tikz,border=3mm]{standalone}
\usepackage[top=3cm, bottom=2.8cm, left=2.5cm, right=0.5cm, marginparwidth=0cm, marginparsep=0cm, headheight=13.6pt, a4paper]{geometry}
\usepackage[T1]{fontenc}
\usepackage{polyglossia}
\setmainlanguage{spanish}
\usepackage{xparse}
\usepackage{tikz}
\usetikzlibrary{arrows.meta, calc}
\tikzstyle{every picture}+=[remember picture]
\usepackage{musixtex}
\begin{document}
Grand staff with piano notes:
\vspace*{2.5cm}
\begin{music}
\parindent10mm
%\sepbarrules
\instrumentnumber{2}
\setstaffs1{1}
\setstaffs2{1}
\setclef20
\setclef16
\smallmusicsize
\curlybrackets{12}
\nobarnumbers
\font\A=phvb8t at 8pt% second fontsize, 4pt possible, too
\startpiece%
\notes\zcn{B}{\hspace*{-0.10cm}\bf\A A0}\wh{!`A}&\en \notes\zcn{B}{\hspace{-0.10cm}\bf\A B0}\wh{!`B}&\en
\notes\zcn{``C}{\hspace{-0.10cm}\bf\A C1}\wh{!C}&\en \notes\zcn{``C}{\hspace*{-0.10cm}\bf\A B1}\wh{!B}&\en \notes\zcn{``H}{\hspace*{-0.10cm}\bf\A C2}\wh{!C}&\en \notes\zcn{``H}{\hspace*{-0.10cm}\bf\A B2}\wh{!I}&\en \notes\zcn{A}{\hspace*{-0.10cm}\bf\A C3}\wh{!J}&\en \notes\zcn{A}{\hspace*{-0.10cm}\bf\A B3}\wh{!b}&\en \notes&\zcn{c}{\hspace{-0.10cm}\bf\A C4}\wh{!c}\en
\notes&\zcn{`c}{\hspace{-0.10cm}\bf\A B4}\wh{!i}\en
\notes&\zcn{f}{\hspace*{-0.10cm}\bf\A C5}\wh{!j}\en \notes&\zcn{f}{\hspace{-0.10cm}\bf\A B5}\wh{!p}\en
\notes&\zcn{`f}{\hspace{-0.10cm}\bf\A C6}\wh{!q}\en
\notes&\zcn{f}{\hspace*{-0.10cm}\bf\A B6}\wh{!w}\en \notes&\zcn{f}{\hspace{-0.10cm}\bf\A C7}\wh{!x}\en
\notes&\zcn{`f}{\hspace{-0.10cm}\bf\A B7}\Ioctfinup 1{''s}\wh{!w}\en
\notes&\zcn{`f}{\hspace*{-0.10cm}\bf\A C8}\Toctfin1\wh{!x}\en
\Endpiece
\end{music}
\end{document}


