It may be that there is nothing to worry about, especially if they have always sounded that way.
If it is a new situation that has arisen however and areas of wall tiles start to sound drummy when tapped there could be an issue that needs investigating.
A drummy tile installation is simply the result of incorrect tiling.
None of the above situations are ideal, and we would always recommend that the tiling be rectified by the builder or tiler responsible. Unfortunately, the only way to fix this sort of problem is to retile the area properly. If it is due to a waterproofing failure, that will also need to be reapplied.
If the tiles sound drummy immediately following installation, however, and if only a few isolated tiles are affected; you may want to be prepared to avoid the convenience of repair work. If so, talk to your builder or tiler and ask for a signed guarantee that they will rectify the issue promptly and at no charge if the tiling system does fail at some future date.
If the installation seemed fine initially, but after some time (usually at least a year) the drumminess occurs, you need to prepare for repair work at your earliest convenience. The tiles may start to fall off the wall, but the damage to the substrate is the critical issue.
Get in touch with your builder or tiler as soon as possible.