* Beautifully - This describes how someone sings in a pleasing way.
* Loudly - This describes how someone sings with a strong volume.
There are many other adverbs you could use depending on the specific way someone sings, such as:
* Softly
* Sadly
* Joyfully
* Melodiously
* Passionately
* Quietly
* Harshly
* Nervously
* Cheerfully