Think about what you are hoping for. A 10dB reduction means reducing the sound power by a factor of 10. Dropping the sound power by 60dB therefore means reducing it by a factor of 1 million, or stopping 99.9999%
To put this in context, this is equivalent to building a wall that blocks 100% of the sound, (let's say the wall is 9' tall by 10' wide), and then leaving a 1/8" diameter hole in it.
See also the wikipedia article on sound transmission class, which suggests that typical interior wall construction gives STC 33, and that STC 45 is sufficient for loud speech to be inaudible. STC 45 can be achieved with double layer of 1/2" drywall on each side of the partition, over wood studs with batt insulation in the wall.
Your question is concerning two things:
1. Proper fitting (wood planks to masonry wall).
2. Proper wood treatment (anti-moisture).
So that's how will my answer look like. Let's look at this.
1. Proper fitting.
I would advise preparing additional support planks going vertically with - say - 1 meter space (that spacing requires additional insight on how much planks will weight, what wall you got and what kind of bolts you will use; use your aptitude and experience or ask a prO). These support planks will be bolted to the wall on all wood-wall height and have only supportive purpose (don't need any beautiful finishing or something). The spacing also determines if (or HOW MUCH) your wood-wall would deform over time. Shorter spaces mean less visible deformations (in short).
Your proper planks - THE planks you want to see as a decorative - will be attached to these supports I mentioned before. That will make support planks perpendicular to these decoratives. Also, I would leave some few centimeters (or, preferably, more) space from the floor to avoid water to be in contact with planks (due to water from shower or cleaning tools) and to let air circulate between masonry wall and wood-wall. Similiar space would be needed up near ceiling.
2. Proper wood treatment.
This will require appropriate info on what you got in stores - all of these chemicals that render your wood water- and moisture-proof (at least for some years...). On that I advise to talk to some shot helper that seem to have proper knowlege.
EDIT concerning question edit :)
The space between masonry and wood is to be kept, I'm afraid. Here are some reasons:
- it allows air (and often steam) to travel op the wall; moisture has an occasion to evaporate
- using supportive planks lets you not to drill that many holes in a wall (cuz you will connect decorative planks to supportive planks with screws)
- it allows planks to deform, avoiding some of tensions that would lead to look bad/destroy it
I'm aware that it may not look as intended with this visible difference between wood and tiles, but you may try either to neutralize that view or use it to your benefit (better visual effect). You can achieve that whatever you like (covering first line of tiles with last line of wood; plaing decorative hangers and so on...).
Of course, You can glue/attach wood to the wall with some glu-ish adhesive, but I guess that wood will 'work' and deform over time. If, later on, You would decide that any refinishing is to be done, it would be easier to unatttach the screws than tearing down (un-glue) all of them.
I would also wait for other DIY-ers to give their opinions as well. Mine is one of many, and I know it :)
Best Answer
You could put a few layers of cement board on the back side of the plywood.
I would use two layers of 1/2 inch cement backer board and use outdoor grade construction adhesive in conjunction with screws.
Start with one layer and see how it sounds.