“Aloud” uses the “a-” preposition 1 meaning, in the situation of loudness.

• 01.059 he said aloud.
• 01.137 and accidentally said it aloud.
• 05.063 he said aloud.
• 06.088 and he found himself saying aloud:
• 12.060 he said aloud.
• 12.067 and Smaug laughed aloud.
• 12.077 exclaimed Bilbo aloud,
• 13.010 he squeaked aloud.
• 13.033 They spoke aloud,
• 13.040 Thorin!’ he cried aloud.

“aloud, adv. and adj.” OED Online. Oxford University Press, March 2015. Web. 18 May 2015.