Change layout of software buttons on home screen
The comment above about replacing System UI is correct. However there's a hack that overlays Android's built-in navigation bar with a "customizable navbar-like panel". The real navbar is hidden but still exists.

If you're still interested in a hard fix you need to unpack, modify and repack SystemUI.apk. This can be done using the apktool utility, which unzips the apk in editable form. In the resulting file tree, look for and edit res/layout/system_bar.xml using a plain text editor. Add the keymap and icons for volume control:

&lt;com.android.systemui.statusbar.policy.KeyButtonView android:id="@id/sub" android:layout_width="@dimen/navigation_key_width" android:layout_height="fill_parent" android:src="@drawable/ic_sysbar_volume_down" systemui:keyCode="25" systemui:glowBackground="@drawable/ic_sysbar_highlight" /&gt;
&lt;com.android.systemui.statusbar.policy.KeyButtonView android:id="@id/add" android:layout_width="@dimen/navigation_key_width" android:layout_height="fill_parent" android:src="@drawable/ic_sysbar_volume_up" systemui:keyCode="24" systemui:glowBackground="@drawable/ic_sysbar_highlight" /&gt;


The exact placement of the code probably depends on the customization done by the ROM vendor. But a likely spot is to search and place it after the line that contains the text "android:src="@drawable/ic_sysbar_recent".

After inserting the references, you need to copy the icons themselves to the right folder:

res/drawable-mdpi/ic_sysbar_volume_down.png
res/drawable-mdpi/ic_sysbar_volume_up_land.png
res/drawable-mdpi/ic_sysbar_volume_down_land.png
res/drawable-mdpi/ic_sysbar_volume_up.png


The final step is to repack the edited system_bar.xml and new icons along with the rest of the unpacked SystemUI.apk.

I'm sure my instructions are incomplete. So remember to keep a good (and tested) backup and recovery tool at hand before attempting to install your hacked apk!