The Debian packaging of volume-el is maintained in git, using the merging
workflow described in dgit-maint-merge(7).  There isn't a patch
queue that can be represented as a quilt series.

A detailed breakdown of the changes is available from their
canonical representation - git commits in the packaging repository.
For example, to see the changes made by the Debian maintainer in
the first upload of upstream version 1.2.3, you could use:

    % git clone https://git.dgit.debian.org/volume-el
    % cd volume-el
    % git log --oneline 1.2.3..debian/1.2.3-1 -- . ':!debian'

(If you have dgit, use `dgit clone volume-el`, rather than plain `git
clone`.)

A single combined diff, containing all the changes, follows.
--- volume-el-1.0+git.20201002.afb75a5.orig/volume.el
+++ volume-el-1.0+git.20201002.afb75a5/volume.el
@@ -452,7 +452,7 @@ This corresponds to the `-D' option of a
 
 (when (fboundp 'define-obsolete-variable-alias)
   (define-obsolete-variable-alias 'volume-amixer-control
-    'volume-amixer-default-channel))
+    'volume-amixer-default-channel "1.0"))
 
 (defvar volume-amixer-current-channel volume-amixer-default-channel
   "The name of the ALSA mixer channel to manipulate.")
@@ -654,7 +654,7 @@ Return either the new volume or nil, dep
 
 (when (fboundp 'define-obsolete-function-alias)
   (define-obsolete-function-alias 'volume-channel-name
-    'volume-channel-label))
+    'volume-channel-label "1.0"))
 
 (defun volume-channels ()
   "Return the list of available channels."
