I have disabled sounds for Reminders, however is it possible to configure a single reminder to play a sound? Otherwise I have to rely on the alarm clock?
Unfortunately that is not possible with the Calendar on Windows Phone itself.
Two points to note:
1- Even if you have multiple Windows Live calendars configured, they are all grouped under "Reminders" and so you cannot even enable sounds for one calendar and disable them for another. However you can enable email reminders for particular calendars.
2- You can try using a third party calendar app to see if this is possible. Here is a discussion about some highly rated calendar apps on Windows phone.