If you want one device for both volume levels, use the arduino to generate the tones and a digital pot IC to control the volume to an amplifier.
An all in one digital volume control and amplifier module is this one
http://www.ebay.com/itm/170850491058?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649
or
http://www.ebay.com/itm/2-2W-4O-PAM8803-Class-D-Audio-Amplifier-Board-Simple-/350604041652?pt=US_Home_Audio_Amplifiers_Preamps&hash=item51a1a11db4