Sometimes The alarm volume is set to high (through Preferences) but the sound is still low, very low (especially compared to the alarm sound on Tungsten T).
In that case user should download a shareware program called VolumeCare that amplifies the so-so "normal" volume of the Treo 650 and also allows you to individually adjust Volume and Boost for Handset, Speakerphone, Headset, etc. Give it a try and see if it works for you.
Here's the web site:
http://GoTreoSoftware.com