So if the warning sound is audible without the hearing protector, it will usually be audible when wearing the hearing protector. For the unusual situations where this is not the case, the solution may be as simple as using a different hearing protector. Also, many warning systems can be adjusted or changed so warning signals are easier to detect. In cases where it's important that the sound just be quieter without any other changes, there are hearing protectors that can provide flat attenuation. There are even protectors that professional concert musicians use that can lower the sound level while retaining sound fidelity.
Some people find they can wear hearing protectors and still understand speech.
Others will have trouble hearing speech while wearing hearing protectors. In some cases, wearing hearing protectors can make it easier to understand speech. In other instances, people may be using protectors that keep out too much sound. You may need a protector that reduces the sound enough to be safe without reducing the sound too much to hear speech at a comfortably loud level. For those people who work in noise and must communicate, it may also be necessary to use communication headsets.