If both the main domain and subdomain point to the same directory on the filesystem, then you need to check the requested host using a mod_rewrite condition.
So, for example, the following directives should go before the WordPress front-controller (ie. before the # BEGIN WordPress section):
RewriteCond %{HTTP_HOST} ^(example\.com) [NC]
RewriteRule ^(music/end/)$ http://music.%1/$1 [R=301,L]
The use of backreferences in the RewriteRule substitution is simply to prevent repetition. %1 is a backreference to the captured group in the last matched CondPattern (ie. example.com) and $1 is a backreference to the captured group in the RewriteRule pattern (ie. music/end/).
You need to use mod_rewrite (as opposed to a mod_alias Redirect) since you most probably have existing mod_rewrite directives associated with WordPress, and there could be conflicts otherwise. Besides, you can’t check the Host header using mod_alias.
You will need to clear your browser cache before testing.
UPDATE: From your comment…
RewriteRule ^ng-music/(.*) https://music.example.com/music/$1 [R=301,L]
This does something different to the “desired” redirect as stated in your question. By itself, this shouldn’t result in a redirect loop.