This is a very common myth that tongue-tie causes unclear speech but actually it is not so. But yes, the other problem i.e. fluid in the ear can be responsible for unclear speech. It so happens that because of presence of fluid , hearing deteriorates and owing to reduced hearing, the child tends to mix up similar sounding letters.
Please get the following done:
1. Audiometry (either Pure Tone or Free Field , depending upon the child's cooperation).
2. Impedence Audiometry.
3. Medication or Aural rehabilitation depending upon the diagnosis.
4. Speech Therapy & Language stimulation.