People are finding more and more interesting applications for wireless speakers. Many of these applications involve streaming from a Bluetooth-enabled device. For that particular purpose, Bluetooth speakers are optimal. However, there are also some other applications such as streaming music outside of your house. For this application, Bluetooth speakers are usually not suitable because of the short wireless range.
The range of Bluetooth speakers is typically less than 50 feet. However, there are some wireless speakers from Amphony available which have much larger operating range. Some models actually can get up to 600 feet and range. For outdoor applications, I would definitely consider one of those alternatives. The fact is that the range which is specified by the manufacturer does usually not apply to situations where the signal is traversing walls and partitions. The reason for that is the signal gets weaker every time it goes through wall. This fact will reduce the actual range tremendously.
Because in most cases, outdoor wireless speakers will be mounted such that the signal cannot reach the speakers without passing through wall, you will need a sufficiently strong transmitter. Also, the transmitter should be situated as close to the speakers as possible. There must be no metal objects between the transmitter and the location where the speakers are mounted. A wireless signal is unable to pass directly through metal.
When mounting speakers outdoors, they have to be resistant to water, humidity and sunlight. You should always pick a location where there is no direct exposure to sunlight and rain. If necessary, I would recommend building and enclosure above the speaker to protect it from rain. Rain creeping into the speaker can do some serious damage. Some speakers are certified for outdoor use and are probably waterproof. However, it never hurts to exercise some protective measures. At the very least, you will prolong the lifetime of you speakers.
Obviously, the mounds that you use for assembling the speakers have to be sturdy and should not rust. The speakers themselves have to be fastened securely inside the mounds. If they were to dislodge from the mounds and fall to the ground, most likely they would be irreparably damaged. Before assembling the speakers in the amounts, I recommend testing for range. That will ensure that the speakers will work as desired. If it turns out that the wireless signal is not strong enough to reach the location of the speakers then you would have wasted valuable time and possibly have done some damage to the walls where the mounds was supposed to be assembled.