If you want to change it across all campaigns, you just need to replace the corresponding sound file in /var/lib/asterisk/sounds
"Your call is important to us" is generic_hold.gsm. You can record your custom wav file in any sound recording software of your choice and convert to gsm by using sox as described in
http://www.voip-info.org/wiki/view/Asterisk+sound+files or you may use the extension 8168 if you have it configured in extensions.conf. 8168 will invoke agi-record_prompts.agi and your file will be saved in the format 851XXXXX. You will have to rename that to generic_hold.gsm.
If you want different campaigns to have different recordings, a quick way to do that is to simply copy the inbound script (say agi-VDADcloser_inboundCID.agi) to camp1.agi; camp2.agi; edit the script and replace generic_hold (line 508 in my version) with the the filename of your choice.
Music on hold is park.gsm - (this is when the agent places the customer on hold using PARK CUSTOMER) SCRATCH_INSTALL describes how this works. In short, you can record your own gsm, create and new extension and configure the campaign to use that.