DJWHT, I like the idea of playing it once only at the beginning. Makes it nice and simple.
If you do want to play the EWT message regularly, however, other things to consider would be:
1) How you trigger the message (based on age of call or call centre intrinsics such as EWT, calls in queue, agents free etc)
2) How many people are likely to hear this (to estimate impact on mail channel usage. I may be wrong, but can't see you being able to do EWT using broadcast)
3) Do you want customers to hear if EWT is actually going up? If not, you will need to script this.
e.g.
4) How accurate will the EWT be (most accurate with high volume call centres with similar call lengths).
See also
5) Based on 4) above, select the EWT ranges. Don't specifically state the EWT as it is unlikely to be accurate. Instead use it to trigger one of a number of messages i.e. 'EWT is less/more than five/ten/fifteen minutes'
6) What do you hope playing the EWT message will achieve? Getting people to hold on or call back later (use lessthan/morethan)
DD