Corn roots can make and sense sound.
The noise they make is very quiet, but if you place a corn seedling in front of a laser, the sound vibrations can be detected and you can play it back through a speaker.
The sound resembles a soft crackling, and is made by the roots growing. This may not sound like vocalizing, but if the sound is played back to a seedling, it responds.
One experiment played the sound to a row of corn seedlings, and timelapse footage clearly showed that the roots were growing towards the sound.
There has been speculation on the purpose of this adaptation, and it seems to be related to direct the roots to grow somewhere, perhaps away from competitors.
Since the noise is made by the roots growing, to have audible vocalizations you would need faster growth. So, let's look at the fastest growing plant in our world, bamboo.
Scientists believe that fast growth in bamboo is caused by high coordination between cell division, cell growth and cell wall biosynthesis, which allows the plant cells to reproduce far quicker and optimizes the plants for rapid growth.
Grasses grow by elongation, but if your roots did a similar thing to bamboo, they would produce louder noises. Different plant species might produce different noises to due to differences in the density, shape etc. of the roots.
Ideally it should be a repeatable sound, not one that requires the regrowth of a cell structure
This could be repeatable if it had the super-fast growth I discussed earlier. The plant could have several offshoots of the stem, the cells of which would grow extremely fast. Differing speeds of growth and lengths of it would produce different sounds, and the offshoot breaks off when it is finished a single "statement". The other plant "hears" this, responds, and the cycle repeats again.
Here is an excellent suggestion from Inoutguttiwutts:
Your answer becomes more viable if the plants are a hive mind. Each growth sound can form a phoneme, and each phoneme comes from a different plant in the hive. Thus individual plants communicate in monosyllabic 'grunts', but a whole plantation 'sings'
Again, an answer may lie in bamboo. Each bamboo plant is in fact interconnected to others by a subterranean system of root-like structures.
The world's heaviest organism, Pando, is a clonal colony of a single male quaking aspen, which looks like an entire forest. It uses a similar setup to bamboo.
Anyway, a plant like Pando could potentially evolve to have many "drones" which grow with extreme speed to vocalize. A single plant in the centre contains a sort of pseudo-nervous system (The system could be based on chemicals contained in the water carried by the xylem) which synchronizes the vocalizations of the drones to produce communicative messages.
Could this develop into speech? If there was a selective pressure to do so, probably. If all the plants grew this fast, there would be higher competition among species for growing space, so more complex communication might be needed. Or, if your plants have other sensory systems besides "hearing", they might be able to detect herbivores, and tell their fellow plants to secrete acid, extend spines or something.