In fact its fun and great to make your own bird sound. You could save some money and a sound that no one has. Provided you know what you are doing, otherwise you are creating a disaster and all you know, few months later you discover that you birds are getting lesser and the numbers of nest is lower.
There are hundreds of free sound editing software on the net where you could download and use it for free or you could purchase it. But do you know what is fighting sound, mating sound, aggressive sound, panic sound,etc? If you do not know, my sincere advise is not worthwhile to even try it unless you are familiar with the sound. Nowadays, all CDs Bird sounds are collaborate with the above to create birds to come to your farm this due to the demand of most farmers mentality the more birds come to the sound the better it is, this is always not the case. I am sure many of you have heard that once the sound is played, many birds come and week by week or months later the number birds reduced. Or even some had experienced that many bird stay but no nest-lah!
Above is the frequency of bird sound between them there are some unfavorable sound. One need to understand it before starting to edit it. Not all parts of the CDs are favorable to birds. In fact, some part really frighten birds away. Remember, Swiftlets have good memory once they labeled your farm,"you farm is history". Imagine, they could find their own nest among thousands of nests.
Currently, synthetic sound has been added into the bird sound too, this is to create a sound that curious to birds so that they would follow the source of sound.
So be careful and know what you are doing.