Let $(v_1, ..., v_{2^n})$ be the vertices of an $n$-dimensional hypercube. Label each vertex $v_i$ with a real number $x_i$. Label each edge of the hypercube with the product of labels of the two vertices it connects. Let $S$ be the sum of the labels of all the edges. Over all possible labelings, find the minimum possible value of $\frac{S}{x^2_1+ x^2_2+ ...+ x^2_n}$ in terms of $ n$.

Note: an $n$ dimensional hypercube is a graph on $2^n$ vertices labeled labeled with the binary strings of length $n$, where two vertices have an edge between them if and only if their labels differ in exactly one place. For instance, the vertices $100$ and $101$ on the $3$ dimensional hypercube are connected, but the vertices $100$ and $111$ are not.
1. **Understanding the structure of the hypercube:**
   - An $n$-dimensional hypercube has $2^n$ vertices.
   - Each vertex can be represented by an $n$-bit binary string.
   - Two vertices are connected by an edge if their binary representations differ in exactly one bit.

2. **Labeling the vertices:**
   - Let each vertex $v_i$ be labeled with a real number $x_i$.
   - The edge between vertices $v_i$ and $v_j$ is labeled with the product $x_i x_j$.

3. **Summing the edge labels:**
   - Each vertex $v_i$ is connected to $n$ other vertices (since flipping any one of the $n$ bits results in a neighboring vertex).
   - Therefore, each $x_i$ appears in $n$ different edge products.
   - The total sum $S$ of the edge labels is given by:
     \[
     S = \sum_{\text{edges } (v_i, v_j)} x_i x_j
     \]
   - Since each edge is counted twice in the sum, we can write:
     \[
     S = \frac{1}{2} \sum_{i=1}^{2^n} \sum_{j \text{ adjacent to } i} x_i x_j
     \]

4. **Simplifying the sum:**
   - Each $x_i$ appears in $n$ terms of the form $x_i x_j$ for each of its $n$ neighbors.
   - Thus, the sum can be rewritten as:
     \[
     S = \frac{1}{2} \sum_{i=1}^{2^n} x_i \left( \sum_{j \text{ adjacent to } i} x_j \right)
     \]

5. **Using symmetry and convexity:**
   - By the Arithmetic Mean-Geometric Mean Inequality (AM-GM), the minimum value of $S$ is attained when all $x_i$ are equal.
   - Let $x_i = c$ for all $i$. Then:
     \[
     S = \frac{1}{2} \sum_{i=1}^{2^n} c \left( \sum_{j \text{ adjacent to } i} c \right) = \frac{1}{2} \sum_{i=1}^{2^n} c \cdot (n \cdot c) = \frac{1}{2} \cdot 2^n \cdot n \cdot c^2 = n \cdot 2^{n-1} \cdot c^2
     \]

6. **Calculating the ratio:**
   - The sum of the squares of the labels is:
     \[
     \sum_{i=1}^{2^n} x_i^2 = \sum_{i=1}^{2^n} c^2 = 2^n \cdot c^2
     \]
   - Therefore, the ratio $\frac{S}{\sum_{i=1}^{2^n} x_i^2}$ is:
     \[
     \frac{S}{\sum_{i=1}^{2^n} x_i^2} = \frac{n \cdot 2^{n-1} \cdot c^2}{2^n \cdot c^2} = \frac{n \cdot 2^{n-1}}{2^n} = \frac{n}{2}
     \]

The final answer is $\boxed{\frac{n}{2}}$