Modify the following variable in /etc/default/grub:
GRUB_INIT_TUNE=“tempo [note_pitch note_duration] [second_note_pitch second_note_duration] …”
Then
# grub-mkconfig -o /boot/grub/grub.cfg

Tunes can be tested here

In /etc/default/grub:
GRUB_DISABLE_SUBMENU=y # show all entries in one menu, instead of submenus
GRUB_DEFAULT=saved
GRUB_SAVEDEFAULT=true # remember last used entry as default
Then
# grub-mkconfig -o /boot/grub/grub.cfg