The sound speed of wood is related to important wood quality properties such as the microfibril angle of the S2 layer in the cell wall, stiffness, and shrinkage propensity. Measuring the sound speed of seedling stems has benefits to the forestry industry, potentially enabling early selection of trees that yield better quality wood. A nondamaging longitudinal-wave time-of-flight (LWToF) acoustic technique was used to determine the sound speed of 10 cm long sections of 2-year-old Pinus radiata D. Don seedlings. The measured sections were harvested and acoustic resonance used to determine the sound speed of the sections before and after the bark was removed and after the remaining xylem was dried. A linear relationship between the acoustic resonance sound speed of the dry xylem and the LWToF sound speed of the seedling stem was found (R2 = 0.89). To demonstrate a potential application using the LWToF acoustic technique, it was used as a tool for investigating the effect of various applied stresses on wood properties of a clone of P. radiata. The LWToF sound speed measurements of phytohormone stressed stems were significantly lower than the control stems, indicating the negative impact on stiffness and shrinkage propensity imposed by this treatment.